Chemical composition -- A hydrated beryllium aluminum silicate.
Color -- Colorless, light green or yellow, light to dark blue, violet.
Optics -- R.I 1.65-1.68. Biaxial positive.
Durability -- Hardness 6.5-7.5. Brittle.
Crystal structure -- Monoclinic.
Specific Gravity -- 2.99-3.10.
Sources -- Brazil, Ural Mountains, Zimbabwe, and a few other locations. Euclase in strong colors or large sizes is quite a rarity and can be quite attractive. While it can be worn (with care) in jewelery, it is best preserved in collections.