DSC_0378_Nephrite jade cabochon
This is an unusually swirled piece of nephrite, with some slight chatoyance. It almost resembles seraphinite, but it's definitely jade.
Nephrite jade has a chemical composition of Ca2(Mg,Fe)5Si8O22(OH)2. When the proportion of iron to magnesium is higher, the jade is darker green. With the amounts of iron and magnesium being variable, all iron yields "black" jade; all magnesium yields white ("mutton fat") jade. In this piece, one can see the extremes and in between.