This is green and purple Fluorite. It can be found in many places around the globe; this happens to be from Mexico. Pure fluorite is colorless, but impurities from other minerals can give it color. The most common colors are purple, blue, green, yellow, or colorless. Less common are pink, red, white, brown, and black. It is the mineral form of calcium fluoride, CaF2, which is completely safe, but the element it is made from, Flourine (F), is one of the most dangerous elements around. One drop of concentrated hydrofluoric acid can kill you – yet it is used in the manufacture of Teflon, and a treatment for syphilis.
Blue-Green Fluorite supposedly has thermo-luminescence, which means that it will glow when heated, even with a low temperature like body heat. None of the specimens I have present that, though.
These stones are generally too soft for jewelry, but they do make for some pretty gemstones. Someday I’ll pick up a faceting machine and practice using these stones.