Dichroism

Dichroism is the specific case of pleochroism in uniaxial crystals — crystals with one optic axis — in which two distinct colors appear along two perpendicular crystallographic directions. The phenomenon is characteristic of the corundum family (sapphire, ruby), the tourmaline family, beryl (emerald, aquamarine, morganite, heliodor), and many other colored stones; it is observed using a dichroscope, a small handheld instrument with a calcite prism that splits an incoming light path into two perpendicular polarisations.

In sapphire and ruby (uniaxial crystals with the optic axis along the c-axis of the corundum structure), the two dichroic colors are the "ordinary ray" color (perpendicular to the c-axis) and the "extraordinary ray" color (parallel to the c-axis). Blue sapphire shows a violet-blue along the ordinary ray and a greenish-blue along the extraordinary ray; the cutter typically orients the stone so the most desired color (violet-blue for top-quality sapphire) faces up.

Ruby shows a strong dichroism that defines the trade's orientation discipline. Top Mogok ruby shows pure red along the ordinary ray and a slightly orangish-red along the extraordinary ray; pigeon-blood color is achieved by cutting the stone so the ordinary-ray color faces up. A misoriented ruby — cut with the extraordinary ray facing up — looks materially different even at the same nominal color grade, and the trade prices misoriented stones at meaningful discounts.

Tourmaline shows dramatic dichroism that varies by color. Indicolite (blue tourmaline) shows blue-green along one axis and dark blue or violet along the other; pink tourmaline shows pink along one axis and pale or yellow along the other; watermelon tourmaline shows pink and green stratification that is partially dichroism and partially zoning. The cutter chooses orientation to maximise the face-up color, but the dichroism is strong enough that the second axis remains visible when the stone is tilted.

Emerald shows weaker dichroism than tourmaline but strong enough to require cutter discipline. The two colors are the deep green ordinary ray and a lighter, more yellow-green extraordinary ray. Top-quality Colombian emerald is cut to present the deep green face-up; the slight yellow shift visible from the side is a normal feature, not a defect.

The dichroscope test is one of the standard gemological tests for stone identification. A non-dichroic stone tested under a dichroscope shows the same color from both perpendicular axes — narrowing the identification to cubic minerals (garnet, spinel) or amorphous materials (glass, plastic) or diamond. A dichroic stone narrows the identification to uniaxial minerals — sapphire, ruby, tourmaline, beryl, zircon, and so on.

Dichroism does not appear on standard lab reports as a graded property; it is observed during the identification process but reported only in descriptive notes if remarkable.