Alexandrite

Alexandrite is the color-changing variety of chrysoberyl, and it requires one of the most improbable geological coincidences in gemology to form. Beryllium (from granitic pegmatite fluids) and chromium (from ultramafic rocks) must both be present in the same environment - but geologically, these elements almost never occur together because they originate from fundamentally different rock types.

The rare situations where beryllium-bearing pegmatite fluids interact with chromium-bearing metamorphic rocks at the right temperature and pressure create the conditions for alexandrite crystallization. The chromium that gets incorporated into the chrysoberyl structure is what produces the color change effect.

The color change works because chromium absorbs light in a narrow yellow band of the visible spectrum while transmitting both red and green. In daylight (which is blue-rich), the transmitted green dominates. In incandescent light (which is red-rich), the transmitted red dominates. Your eyes and brain perceive the stone as changing color, even though the stone itself hasn't changed at all - only the light illuminating it.

Alexandrite is identified by its dramatic color change - green to blue-green in daylight, red to purplish-red in incandescent light. The strength of the color change is the primary quality factor. At hardness 8.5, alexandrite is extremely durable (harder than anything except diamond, sapphire, and ruby).

Distinguish from color-change garnet (lower hardness, different color change range), color-change sapphire (different color change characteristics), and synthetic alexandrite (very common, requires lab testing). The combination of exceptional hardness and strong color change is diagnostic.