From Dwarf Fortress Wiki
Alexandrites are semi-precious gems found as small clusters in granite, marble, and schist layers. In the real world, alexandrite is a variety of chrysoberyl, and is commonly used in lasers for laser hair removal.
[INORGANIC:ALEXANDRITE] [USE_MATERIAL_TEMPLATE:STONE_TEMPLATE] [MATERIAL_VALUE:20][DISPLAY_COLOR:5:7:1][TILE:15][IS_GEM:alexandrite:STP:OVERWRITE_SOLID] [ENVIRONMENT_SPEC:GRANITE:CLUSTER_SMALL:100] [ENVIRONMENT_SPEC:SCHIST:CLUSTER_SMALL:100] [ENVIRONMENT_SPEC:MARBLE:CLUSTER_SMALL:100] [SOLID_DENSITY:3730] Common to chrysoberyls. Range 3680 - 3780 [STATE_COLOR:ALL_SOLID:VIOLET] Occurs in metamorphic layers per this: http://www.emporia.edu/earthsci/amber/go336/berg/tanzanite.html Of course, it should also be really rare, because it only occurs in one part of the world.