Green crystal. This specimen is opaque, but good samples are transparent.
Chrysoberyl is the third hardest gem after diamond and corundum.
There is a form of chrysoberyl called alexandrite. In daylight, it is green, but under artificial light it turns purple. It is said to be "an emerald by day and an amethyst by night". The two pictures above were taken under natural and artificial light (camera flash). It's a poor specimen, so the artifical light makes it look very dark.
Another sort of chrysoberyl is called a cymophane. It has a silky lustre.
