Diamond & Gemstone Lexicon

The vocabulary the desk uses every day — crown angle, table percentage, chevron facets, the fluorescence grades, the country-of-origin terms for emerald, ruby and sapphire, and the grading and origin vocabulary GIA, AGS, IGI, SSEF, Gübelin, AGL and GRS issue on their reports. Every term has a stable anchor; internal links across the site point here.

Edited by CaratWire Editorial Desk · Senior gemological editor · Last updated 2026-06-20

Diamond cut & proportions

The proportion vocabulary that governs how a round brilliant returns light. These terms appear on every GIA, AGS and IGI report and determine whether a stone earns a top cut grade or sits one notch down.

Crown angle: The angle between the bezel facets and the table plane on a round brilliant, measured in degrees. GIA Excellent rounds typically sit 33.5–35.5°; outside roughly 32–36° the cut grade drops. Crown angle pairs with pavilion angle to determine light return — a steep crown and shallow pavilion (or the reverse) bleeds light through the bottom.
See: Diamond cut grade explained · GIA cut grade comparison
Pavilion angle: The angle between the pavilion main facets and the girdle plane on the bottom half of a round brilliant. GIA Excellent rounds sit 40.6–41.0°; deviations under 40.0° or over 41.8° break light return and push the cut grade down. Pavilion angle is the single most sensitive proportion in the brilliance equation.
See: Diamond cut grade explained
Table percentage: The width of the flat top facet expressed as a percentage of the average girdle diameter. GIA Excellent round brilliants run 53–58%; AGS Ideal-0 rounds run 53–57%. Larger tables (60%+) trade fire for spread; smaller tables (below 53%) trade spread for fire.
See: Diamond cut grade explained
Depth percentage: Total height of a round brilliant divided by average girdle diameter, expressed as a percentage. GIA Excellent rounds run 59.0–62.6%; deep stones (above 63%) hide carat weight under the girdle and face up small for their weight, shallow stones (below 58%) face up large but leak light through the pavilion.
Girdle: The narrow band where the crown meets the pavilion, and the part of the stone the setting grips. GIA grades girdle thickness from Extremely Thin to Extremely Thick; Thin to Slightly Thick is ideal. An Extremely Thin girdle chips at the setting; Extremely Thick hides carat weight under the band.
Culet: The tiny facet (or point) at the very bottom of the pavilion. Modern GIA reports grade it None, Very Small, Small, Medium, Slightly Large, Large, Very Large, or Extremely Large; None to Small is standard for modern brilliants. A Medium or larger culet is visible through the table as a dark spot.
Chevron facets: V-shaped pavilion facets pointing toward the keel on princess, radiant and some cushion cuts. Chevron count (2, 3 or 4 chevrons) controls the crushed-ice vs. broad-flash look — more chevrons produce finer scintillation and a more diffused appearance, fewer chevrons produce larger flashes.
See: Diamond shapes guide
Bezel facets: The eight large kite-shaped facets on the crown of a round brilliant that meet the table on one side and the girdle on the other. Bezel facets carry most of a round brilliant's fire because they tilt incoming light at the steepest angle on the crown.
Star facets: The eight small triangular facets between the table and the bezel facets on a round brilliant. Their length (expressed as star length %, typically 50–60% on a well-cut round) tunes scintillation pattern at the center of the stone.
Upper-half facetsalso: Upper girdle facets: The sixteen small triangular facets on the crown between the bezel facets and the girdle. Together with the lower-half facets across the girdle, they control the scintillation pattern around the perimeter of the stone.
Lower-half facetsalso: Lower girdle facets: The sixteen small facets on the pavilion side of the girdle. Lower-half length (typically 75–82% on a well-cut round brilliant) is one of the proportions an AGS Ideal-0 grade is most sensitive to.
Triple Excellent (3EX): A round brilliant graded Excellent for Cut, Polish and Symmetry on a GIA report. 3EX is the GIA top tier but does not specify proportion — two 3EX stones can look very different in light return. AGS Ideal-0 (the eight-zero on every axis) is a tighter grade.
See: GIA cut grade comparison · GIA vs. AGS grading
Hearts & Arrows: A visual pattern of eight symmetric arrows (face-up) and eight symmetric hearts (face-down) seen through a viewer in round brilliants cut to exacting symmetry. Hearts & Arrows is a symmetry-precision claim, not a GIA or AGS grade — it requires a separate H&A scope image to verify.
AGS Ideal-0: The top tier on the American Gem Society Performance Cut Grade scale — zeroes on Light Performance, Polish, Symmetry, and Proportion. The AGS scale judges actual light performance via ray tracing rather than averaged proportion buckets, which is why AGS Ideal-0 is a tighter claim than GIA Excellent Cut.
See: GIA vs. AGS grading
Bow-tie effect: A dark hourglass-shaped shadow across the center of fancy elongated cuts (oval, marquise, pear). Caused by light leakage under the viewer's head; severity ranges from barely visible to opaque. Every oval has some bow-tie — buy by photo, not by claim.
See: Diamond shapes guide
Brilliancealso: Light return: The total amount of white light a diamond returns to the eye. Brilliance is a function of crown and pavilion angles, table size, and polish — the proportions on the GIA report predict it directly.
Firealso: Dispersion: The flashes of spectral color a diamond throws when white light is split into its component wavelengths by the facets. Steeper crowns and smaller tables increase fire at the cost of spread.
Scintillation: The pattern of bright and dark flashes a diamond produces when it, the viewer, or the light source moves. Scintillation reads as "sparkle" — well-cut rounds show large, balanced contrast flashes rather than busy or washed-out patterns.
Crushed-ice look: A fine, frosted scintillation pattern characteristic of radiant cuts and some cushion cuts with high chevron counts. Some buyers prefer crushed ice for its low-contrast glow; others read it as "no broad flash" and prefer a step-cut or fewer-chevron cushion.

Diamond color

The GIA D-to-Z color scale and the vocabulary around the master stones, face-up grading, and the separate fancy color scale used for color outside D-Z.

D color: The top of the GIA colorless scale — completely colorless when viewed table-down against a white background. D is rare and carries a premium of 10–25% over E and 20–40% over G at the same clarity and cut.
Colorless range (D–F): GIA grades D, E and F. Under controlled lab conditions an experienced grader can separate them; in a ring, in a jewelry-store light box, nobody can. D-F earns its premium on paper, not face-up.
Near-colorless range (G–J): GIA grades G, H, I and J. Face-up indistinguishable from colorless in most settings; the sweet spot for value-conscious buyers. J shows a faint warmth in larger stones (2 ct+) viewed table-down.
See: Which of the 4Cs matter most
Faint to light range (K–N+): GIA grades K, L and M are Faint; N through R are Very Light; S through Z are Light. Below J the warm tint becomes visible face-up, especially in white-metal settings. K can look "warm white" and pair well with yellow gold; below K the discount is real but the look is yellow.
Master stones: A graded set of reference diamonds GIA graders compare against to assign a color grade. Master stones are graded against a master master set held at GIA headquarters. Color grading is a comparative exercise, not an absolute measurement.
Face-up color: How a diamond looks in a ring, viewed from above — versus the GIA standard, which is table-down against a white background. Cut returns more light face-up, which masks color; a well-cut J face-up reads whiter than a poorly-cut H.
Fancy color diamond: A diamond with color outside the D-Z scale (yellow, brown, pink, blue, green, orange, purple, gray, black, or color combinations). Fancy color diamonds are graded on a separate GIA scale by hue, tone and saturation rather than the absence of color.
See: Fancy color diamonds guide
Fancy Vivid / Fancy Intense / Fancy Deep: The top three GIA saturation grades for fancy color diamonds. Fancy Vivid is the most prized for warm colors (yellow, orange) and pinks; Fancy Deep can read darker but earns premiums on blues and grays. A grade jump from Fancy Intense to Fancy Vivid can double per-carat price.
Hue / tone / saturation: The three axes GIA grades fancy color diamonds on. Hue is the dominant color (Fancy Yellow vs. Fancy Brownish Yellow). Tone is the lightness or darkness. Saturation is the strength of the color — the axis the Fancy Light / Fancy / Fancy Intense / Fancy Vivid / Fancy Deep grades sit on.

Diamond clarity

The eleven-grade GIA clarity scale and the vocabulary used on the plot diagram and key-to-symbols.

FL (Flawless): No internal or external characteristics visible at 10× magnification under standard GIA lighting. FL is rare — less than half of one percent of GIA-graded gem-quality rough finishes Flawless.
IF (Internally Flawless): No internal characteristics at 10×, but minor surface blemishes (extra facets, naturals, polish lines) that do not affect durability or appearance. IF and FL trade within 5% of each other.
VVS1 / VVS2 (Very, Very Slightly Included): Inclusions extremely difficult to see at 10× — pinpoints, internal graining, tiny clouds. VVS1 inclusions usually need a skilled grader several seconds to locate; VVS2 inclusions are slightly easier. Both face up indistinguishable from IF.
VS1 / VS2 (Very Slightly Included): Inclusions visible to a skilled grader at 10× with effort — small clouds, crystals, feathers off-center. VS1 inclusions take some hunting; VS2 inclusions are easier to find but still safely eye-clean in most rounds 1 ct and under.
SI1 / SI2 (Slightly Included): Inclusions noticeable to a skilled grader at 10× and sometimes visible to the eye. SI1 in a round brilliant is often eye-clean; SI2 carries real eye-visibility risk in step cuts (emerald, Asscher) where the open table shows every crystal.
See: Eye-clean diamond clarity
I1 / I2 / I3 (Included): Inclusions obvious at 10× and visible to the unaided eye, sometimes affecting transparency or durability. I1 stones can still be beautiful if the inclusions sit under bezels; I2 and I3 trade as commercial-grade only.
Eye-clean: No inclusions visible to the unaided eye at normal viewing distance (about 30 cm), face-up. Not a GIA grade — a real-world claim that should be verified from a video or in person. SI1 rounds are often eye-clean; SI1 emerald cuts often are not.
See: Eye-clean diamond clarity
Plot diagram: The to-scale drawing on a GIA report showing the location, type and size of clarity characteristics. Inclusions are plotted in red, blemishes in green; the key-to-symbols decodes each plot mark.
See: How to read a GIA report
Pinpoint: A tiny crystal inclusion appearing as a dot at 10×. Pinpoints are the most common VVS-grade characteristic and have no effect on face-up appearance.
Cloud: A group of three or more pinpoints clustered tightly enough to read as a haze. A small cloud noted in the plot has no impact; a "clarity grade based on clouds not shown" comment on the report means the stone may face up milky.
Feather: A fracture inside the diamond. Most feathers are durability-safe in protected settings; feathers that reach the surface or sit on the girdle in a tension-set or four-prong design carry chipping risk.
Crystal: A mineral inclusion (often diamond or garnet) inside the stone. Color matters — a dark crystal in the table is the inclusion most likely to be eye-visible at SI grades.
Key to symbols: The legend on a GIA report that names each plotted clarity characteristic. The order in the key reflects grade-setting weight — the first symbol listed is the one driving the grade.

Diamond fluorescence

How a diamond responds under longwave UV light, the GIA strength grades, and when fluorescence helps versus hurts.

None: No detectable response under longwave UV (365 nm). Roughly 65% of GIA-graded natural diamonds. None is the grade the market prefers in D-H colors because there is no fluorescence-related haze risk at any percentage.
See: Diamond fluorescence explained
Faint: Barely perceptible blue glow under longwave UV. No measurable effect on face-up appearance or value in either direction — Faint trades at the same price as None.
Medium: Clear blue glow visible to a skilled grader under longwave UV. In I-M colors, Medium Blue can warm a yellowish stone toward white face-up and lift the apparent color by half a grade. In D-G the trade discounts Medium 5–15%.
See: GIA fluorescence explained
Strong: Bright blue glow under longwave UV. In I-M colors, Strong Blue can mask the body color enough to lift face-up appearance by a full grade. In D-G the trade discounts Strong 10–25% on milky-risk worry; about 3% of Strong-fluorescence stones actually face up hazy in sunlight.
Very Strong: Intense blue glow under longwave UV. Discounts of 15–35% in D-H colors; small premium possible in K-M if no haze. Higher haze-risk percentage than Strong — buy only from video in daylight, not under jeweler lighting.
Blue fluorescence: The dominant fluorescence color in natural diamonds (over 95% of fluorescent stones). Caused by trace nitrogen aggregates in Type Ia diamonds. Yellow, orange and white fluorescence exist but are rare.
"Milky" / "oily" / "hazy": Trade terms for the cloudy face-up appearance some Strong and Very Strong Blue fluorescent diamonds show in UV-rich light (daylight, especially near a window). Not all Strong-fluorescent stones are milky; the only reliable test is daylight examination.
See: Diamond fluorescence explained
Longwave UV (365 nm): The wavelength GIA uses to grade fluorescence. The blacklight in a jeweler's booth is usually longwave UV; sunlight contains both longwave and shortwave UV, which is why fluorescence behavior outdoors can differ from in-store.

Lab-grown diamond vocabulary

The growth methods, post-growth treatments, and report types that separate one lab-grown stone from another. Lab-grown is a different supply chain from natural — the vocabulary is different.

CVD (Chemical Vapor Deposition): Lab-grown method that builds diamond crystal layer by layer from a methane-hydrogen plasma at relatively low pressure. CVD is the dominant lab-grown method for gem-quality material; growth runs take 2–4 weeks.
See: CVD vs. HPHT lab diamonds
HPHT (High Pressure High Temperature): Lab-grown method that grows diamond from a carbon-saturated metal solvent at pressures and temperatures replicating natural diamond formation. HPHT is the older synthesis route and is still used for industrial grit and for color treatment of finished CVD stones.
See: CVD vs. HPHT lab diamonds
As-grown: A lab-grown diamond that has not received any post-growth color or clarity treatment. As-grown CVD stones often start with a brown tint that HPHT post-treatment removes; an as-grown D-color CVD without HPHT treatment is rare and commands a small premium.
Post-growth HPHT treatment: A high-temperature anneal applied to a finished CVD lab-grown diamond to remove brown tint and shift color toward D-F. Disclosed on IGI and GIA Laboratory-Grown Diamond Reports as "Post-growth treatment: HPHT" — not a defect, but worth knowing.
Diamond seed plate: The thin slice of diamond a CVD reactor grows on. The seed dictates crystal orientation and shows up as a faint plane on a DiamondView image — one of the routes a lab uses to confirm CVD origin.
IGI Laboratory-Grown Diamond Report: The grading report most lab-grown diamonds carry in the US retail channel. IGI grades color, clarity and cut on the same letter and number scale as natural diamond reports, with the lab-grown methodology disclosed and the growth method (CVD or HPHT) noted.
See: IGI vs. GIA lab-grown grading
GIA Laboratory-Grown Diamond Report: GIA's report for lab-grown stones. GIA grades lab-grown on the same D-Z and Flawless-to-I3 scales as natural since 2020, with the growth method, any post-growth treatment, and a laser-inscribed report number. GIA LGD reports trade at a small premium to IGI in the US.

Sapphire vocabulary

Country-of-origin terminology, color call-outs, and treatment vocabulary that move sapphire prices. Origin is roughly half the value of an investment-grade sapphire; the named labs that issue origin opinions are SSEF, Gübelin, AGL and GRS.

Kashmir: Velvety cornflower-blue sapphires mined briefly from the Padar valley (4,500m elevation, northern India) in the 1880s–1930s. Closed deposit. Investment-grade 3–5 ct unheated Kashmir stones with SSEF or Gübelin paper trade $50,000–$250,000+/ct. Any "Kashmir" claim without a top-tier origin report is unfunded.
See: Kashmir sapphire — origin guide
Burma (Mogok): Sapphires from the Mogok Stone Tract in Myanmar. Mogok blues sit a step under Kashmir at the top of the market; clean unheated Mogok blue sapphires with origin paper trade $10,000–$40,000/ct at 3 ct.
Ceylon (Sri Lanka): Sri Lankan sapphires, mined principally around Ratnapura and Elahera. Ceylon is the largest source of fine commercial blue sapphires and the dominant source of padparadscha. Heated Ceylon blues 3 ct trade $1,500–$6,000/ct; unheated fine-quality $8,000–$20,000/ct.
Madagascar (Ilakaka / Andranondambo): Major modern source. Ilakaka produces commercial blues that respond well to traditional heat; Andranondambo produces deeper blues sometimes confused with Mogok material. Origin reports separate Madagascar from Ceylon and Mogok using trace-element chemistry.
Australia: Inky, iron-rich blues from Queensland and New South Wales. Australian sapphires are the lowest-priced major source for clean blues — $200–$1,500/ct at 3 ct — because the iron content gives a dark face-up that the trade discounts.
Thailand / Cambodia: Sapphires from the Thai-Cambodian basalt belt (Chanthaburi, Pailin). Historically the source of many "Burma" claims before lab origin testing matured. Thai and Cambodian blues run iron-rich and trade closer to Australian than to Mogok.
Padparadscha: Pinkish-orange sapphire — the only color call in the sapphire family with its own name. Strictest definition (SSEF, Gübelin): a balanced blend of pink and orange, no dominant hue. Sri Lanka is the traditional source; Madagascar and Tanzania now produce commercial padparadscha. Fine unheated 2 ct padparadscha trades $8,000–$25,000/ct.
"Cornflower blue": Trade name for the velvety medium-blue tone associated with Kashmir sapphire. The term is used loosely outside the trade; on an SSEF or Gübelin report it is a color description, not an origin call.
"Royal blue": Trade name for a deeper, more vivid blue with a slight violet modifier — the color most associated with top Mogok sapphire. SSEF and Gübelin use "vivid blue" or "royal blue" as a color call only when saturation and tone meet their internal reference standards.
Silk: Microscopic rutile needles in sapphire that scatter light and create the velvety character of fine Kashmir and Mogok stones. Heat treatment dissolves silk and improves transparency at the cost of the velvety look — which is why unheated stones command the premium.
No-heat / Unheated: Sapphire that has never been thermally treated to improve color or clarity. Disclosure required on any SSEF, Gübelin, AGL or GRS report. Unheated trades at multiples of heated at investment-grade quality — at commercial grades the premium is smaller because heat is the industry default.
See: Sapphire buying guide
Traditional heat: Heat treatment at 1,400–1,800°C in a controlled-atmosphere furnace to dissolve silk, improve color and clarity. Stable and undetectable to the eye; required to be disclosed on grading reports. The default treatment for commercial sapphire — over 95% of sapphire on the market is heated.
Beryllium diffusion (Be-diffused): Heat treatment with beryllium powder that penetrates the lattice and creates orange/yellow color from the outside in. Disclosed as "Heated + Be" on lab reports. Beryllium-diffused padparadscha and orange sapphire trade at a fraction of natural-color equivalents.
Lattice diffusion: A broader category covering treatments where chemical elements (titanium, beryllium) are introduced into the corundum lattice at high temperature to create or deepen color. Disclosed on reports; values sit well below comparable untreated material.

Ruby vocabulary

Country-of-origin terminology, the "pigeon blood" color call, and the treatments — heat, glass-filling, flux-healing — that determine whether a stone trades at investment or commercial levels.

Mogok (Burma): The historical source of the finest rubies — vivid red with a slight blue undertone, low iron content, fluorescent under daylight. Investment-grade 3 ct unheated pigeon-blood Mogok with Gübelin or SSEF origin paper trades $40,000–$120,000/ct.
See: Burma ruby — origin guide
Mong Hsu (Burma): A separate Burmese ruby source discovered in 1992. Mong Hsu material is typically smaller and frequently flux-healed during heating to seal fractures. Reports distinguish Mong Hsu from Mogok and trade accordingly — Mong Hsu commands a Burma premium but well below Mogok.
Mozambique (Montepuez): The dominant modern ruby source — Montepuez, in Cabo Delgado province. Mozambique rubies cover the full quality spectrum, with the finest material approaching pigeon-blood color. Clean unheated 3 ct Mozambique pigeon-blood with origin paper trades $8,000–$25,000/ct.
Madagascar: Significant secondary source. Madagascar produces both clean unheated material from Andilamena and heavily included pinkish-red ruby from Vatomandry; origin reports separate Madagascar from Mozambique and Burma using trace chemistry.
Thailand (Chanthaburi): Iron-rich, darker rubies historically heated and traded through the Chanthaburi cutting district. Thai rubies trade at $500–$3,000/ct commercial through $5,000–$15,000/ct for cleaner clean stones — typically a tenth of comparable Mogok.
Tajikistan: Pamir mountain ruby — a recent commercial source, often fluorescent and sometimes compared to Mogok material on color, though origin reports separate them on inclusion fingerprinting.
Greenland: A small but actively marketed modern source. Greenland rubies are typically smaller and trade on traceability and ethical-sourcing claims rather than competing with Burma or Mozambique on color.
"Pigeon blood": The trade color call for the finest ruby — a pure vivid red with very slight blue undertone, strong saturation, medium-dark tone, and no brownish or orange modifier. SSEF and Gübelin use "pigeon blood" as a formal color call only when the stone meets their internal reference standards; GRS uses "Pigeon Blood (GRS-type)" with explicit criteria. Without an origin-and-color report the term is just sales language.
See: Ruby buying guide
Glass-filled ruby (lead-glass filled): Heavily fractured low-grade corundum infused with high-lead-content glass to make cracks invisible and the stone appear clean. Sold openly at gem shows for $20–$200/ct; not a treatment a ruby buying for value should accept. Disclosed on every reputable lab report.
Flux-healed: Ruby heated with a borax-based flux that flows into open fractures during cooling and partially heals them. The flux residue is detectable under microscope; reports disclose "heated, evidence of healed fractures with foreign residue". Common in Mong Hsu material; trades below clean unheated but above glass-filled.
Heated (no residue): Ruby thermally treated at 1,400–1,800°C to improve color and dissolve silk, with no fillers added. Standard, stable, disclosed on every reputable report. The market default for commercial ruby — about 95% of ruby in retail channels is heated.

Emerald vocabulary

Country-of-origin terminology, the oil treatment scale every emerald is graded against, and the trade vocabulary around clarity (jardin, gota de aceite) that doesn't appear in other gemstone families.

Colombia (Muzo / Chivor / Coscuez / La Pita): The premier emerald source. Muzo and Chivor are the two historical mines whose names carry the strongest premium; Coscuez and La Pita are productive modern mines. Investment-grade 3 ct minor-oil Muzo with Gübelin or SSEF paper trades $15,000–$50,000+/ct.
See: Colombian emerald — origin guide
Brazil (Capoeirana / Itabira / Belmont): Major modern source. Brazilian emeralds often show a slightly yellow-green hue versus Colombian blue-green and typically trade at 30–60% discount to Colombian at comparable grade.
Zambia (Kagem): Dominant modern source by volume. Zambian emeralds are cleaner and bluer than Colombian or Brazilian, with higher iron content giving a slightly steely undertone. Fine 3 ct Zambian trades $4,000–$12,000/ct.
Afghanistan (Panjshir): A boutique source producing emerald with a color very close to fine Colombian. Panjshir material trades close to Colombian on the rare clean top-color stones; supply is constrained by access.
Ethiopia (Shakiso): Emerging modern source discovered in 2016. Ethiopian emeralds trade between Zambian and Colombian on color — the finest can rival Colombian at a discount; origin reports increasingly include Ethiopia as a distinguishable source.
Russia (Ural Mountains): Historical source — the Mariinsky mine in the Urals. Russian emerald is rarely on the modern market in significant size; older estate pieces with credible Ural provenance carry a small historical premium.
Minor oil (insignificant): The lowest treatment grade SSEF, Gübelin and GRS issue on emerald — only trace amounts of oil detectable in surface-reaching fissures. The grade an investment-grade emerald aspires to. Trades at a multiple of moderate-oil at the same quality.
See: Emerald buying guide
Moderate oil: Standard commercial treatment level — oil clearly present in fissures but not heavily filling. The middle of the SSEF / Gübelin / GRS three-grade scale. Trades 20–50% below minor-oil at comparable quality.
Significant oil: Heavy filling; oil substantially present in fractures. The bottom of the SSEF / Gübelin / GRS scale. Significant-oil stones trade at large discounts and carry maintenance risk — oil dries out and fractures become visible.
Cedarwood oil: The traditional natural oil used to fill emerald fissures. Cedarwood oil has a refractive index close to emerald and is accepted by the trade as the disclosed standard. Synthetic resins (Opticon, Excel, ExCel) are also used and must be disclosed separately.
Opticon / Excel / ExCel: Brand-name epoxy resins used as alternatives to cedarwood oil for filling emerald fissures. Resins are more durable than oil but harder to reverse and must be disclosed by name on grading reports. Resin-treated emerald typically trades below oil-treated at the same visible quality.
Jardin: Trade term (French for "garden") for the network of inclusions characteristic of emerald — three-phase inclusions, growth tubes, fingerprint patterns. A clean emerald is unusual; "no jardin" claims should be verified at 10× because they often mean heavy filler.
Gota de aceite: "Drop of oil" — a rare optical phenomenon in the finest Colombian emerald: a roiled, dappled internal pattern visible at 10× that signals slow crystal growth and top color. Gübelin sometimes notes gota de aceite on origin reports; the term commands a meaningful premium.

General treatments vocabulary

Treatments that apply across multiple stone families and the disclosure language reputable labs use.

HPHT (treatment): High-pressure high-temperature treatment, applied to natural diamonds to remove brown body color and shift color toward D-F. Disclosed on GIA reports as "HPHT processed" with a laser inscription. HPHT-treated naturals trade at 30–50% below untreated equivalents at the same finished color.
See: Treated diamonds to avoid
Irradiation (for color): Electron, neutron or gamma-ray bombardment of a diamond to create color — blue, green, black, brown. Often followed by annealing. Stable but disclosed; irradiated colored diamonds trade at a fraction of natural-color fancy diamonds.
Annealing: Controlled heating after irradiation or as a stand-alone treatment to shift induced colors toward yellow, orange or pink. Always disclosed in conjunction with the originating treatment.
Coating: A thin colored film applied to the surface of a diamond or gemstone to alter face-up color. Unstable — wears and chips. Detectable at 10× and on every reputable lab report; coated stones trade at commercial-grade prices regardless of underlying material.
Glass-filling (fracture-filling): High-lead-content or borosilicate glass injected into surface-reaching fractures to make them invisible. Common on low-grade ruby and on heavily included diamond. Reduces structural integrity; disclosed on every reputable report.
Lattice diffusion (general): Heat treatment with chemical additives (beryllium, titanium) that penetrate the crystal lattice and shift color from the outside in. Used principally on corundum (sapphire, ruby). Disclosed on reports — trades well below comparable untreated material.
Oiling (emerald): The standard emerald clarity treatment — cedarwood or synthetic oil drawn into surface-reaching fissures under vacuum. Graded by the major labs at three levels (insignificant / minor, moderate, significant). Required to be disclosed.
Waxing: Paraffin or other wax applied to porous stones (turquoise, certain opal) to improve surface luster and stability. Disclosed on lab reports; waxed turquoise sits above stabilized but below natural untreated material on price.

Lab-issued grading & origin reports

The grading and origin laboratories cited by name across the site. The lab that issued the report — and what kind of report the lab issues for the stone in question — is half the price signal for any high-value diamond or colored stone.

GIA (Gemological Institute of America): The reference grading authority for diamond. GIA grades color D-Z, clarity FL-I3, cut Excellent-Poor on round brilliants, with a separate fancy color scale. GIA also issues colored stone identification and origin reports on select material; for colored-stone origin on the highest-value stones, the trade defers to SSEF, Gübelin, AGL and GRS.
See: How to read a GIA report
AGS (American Gem Society) — Cut Grade 0: AGS Laboratories pioneered light-performance-based cut grading. AGS's Performance Cut Grade uses ray tracing to score Light Performance, Polish, Symmetry and Proportion 0-10; AGS Ideal-0 (zero on every axis) is a tighter claim than GIA Excellent Cut. AGS Laboratories merged its grading operations into GIA in 2022; the AGS Ideal-0 reference standard remains the cut benchmark.
See: GIA vs. AGS grading
IGI (International Gemological Institute): The dominant grading laboratory for lab-grown diamond and for commercial-grade natural diamond, headquartered in Antwerp. IGI uses the same D-Z and FL-I3 scales as GIA. IGI lab-grown reports trade close to GIA LGD reports in the US retail channel; IGI natural reports trade at a small discount to GIA at the same paper grade.
See: GIA vs. IGI vs. AGS
SSEF (Swiss Gemmological Institute): Swiss origin and treatment authority for colored gemstones, based in Basel. SSEF origin opinions on ruby, sapphire and emerald are accepted by Christie's, Sotheby's, Bonhams and Phillips for high-value lots. Origin call based on trace-element chemistry (LA-ICP-MS) plus inclusion-fingerprint comparison against SSEF's reference population.
See: Reading gemstone lab reports
Gübelin Gem Lab: Swiss gemological laboratory in Lucerne; together with SSEF, the most trusted origin authority for colored gemstones at the top of the market. Gübelin issues full reports with detailed origin and treatment disclosure and provides supplementary opinions ("pigeon blood", "royal blue", "gota de aceite") on material that meets internal reference standards.
GRS (GemResearch Swisslab): Swiss colored-stone laboratory with offices in Lucerne and Bangkok. GRS reports are accepted at auction; GRS uses defined color-call language ("Pigeon Blood (GRS-type)", "Royal Blue") with explicit criteria. Active in Mozambique ruby and modern sapphire sources.
AGL (American Gemological Laboratories): New York-based colored gemstone laboratory issuing identification, treatment and origin reports. AGL is the US-side colored-stone authority most cited at Sotheby's and Christie's New York for ruby, sapphire and emerald. AGL's reports include detailed enhancement grading on a numerical scale that the trade uses to price specific stones.
HRD (Hoge Raad voor Diamant): Belgian diamond grading authority — the European counterpart to GIA, headquartered in Antwerp. HRD reports use the same D-Z color and FL-I3 clarity scales. HRD reports trade close to GIA in the European retail channel; in the US they trade at a small discount.
Diamond type (Ia / Ib / IIa / IIb): A physics-based classification of diamond by nitrogen and boron impurity content. Type IIa (less than 0.001% nitrogen) is the rarest and includes most top D-color stones and most CVD lab-grown. Type Ia (the majority of natural diamond) is nitrogen-bearing; Type IIb (boron-bearing) is the blue family. Tested by FTIR spectroscopy.
LA-ICP-MS: Laser ablation inductively coupled plasma mass spectrometry — the technique SSEF, Gübelin, AGL and GRS use for trace-element chemistry on colored stones. Quantifies the parts-per-million signatures (titanium, iron, gallium, vanadium, magnesium, chromium) that separate Kashmir from Mogok from Madagascar sapphire. The technique that turned origin from opinion into measurement.
DiamondView: Shortwave UV imaging device used by GIA and IGI to separate natural from lab-grown diamond. Natural Type Ia diamond fluoresces blue under DiamondView; CVD lab-grown fluoresces orange or red and shows characteristic horizontal growth lines from the seed plate.
M-Screen / SYNTHdetect: Screening instruments used by diamond grading laboratories and dealers to flag stones that need full lab-grown testing. De Beers' SYNTHdetect uses fluorescence imaging; HRD's M-Screen uses spectroscopy. A "pass" routes the stone to standard grading; a "refer" triggers full DiamondView or FTIR analysis.
See: Reading a GIA laser inscription

How to use this lexicon

Every term has a stable URL — copy the anchor from any heading and the link will resolve directly into that definition. Articles across the site link here whenever they introduce a piece of grading or origin vocabulary, so the lexicon is the canonical reference rather than a separate parallel definition in each guide.

Three places to go next, depending on what you are buying for:

2026 diamond price-per-carat index — what the vocabulary above actually costs, by shape, weight and grading lab.
Colored gemstone origin guide — the four origins (Kashmir, Burma, Colombia, Paraíba) that move colored-stone pricing and the named-lab reports that authenticate them.
Buying guides — where to shop, how to read a grading report, negotiating, appraisals, and insurance.