Dark blueish-green vivianite crystals partially filling a cavity inside a fossil shell of a bivalve (last picture taken with extra light).
Found at the classic location in Kerchenskoye Deposit, Kerch, Crimea, Ukraine
This is a relatively large shell, measuring 132 grams. Size is 42 x 70 x 58 mm.
It is not often you will see this material on the market nowadays. My father aquired this one in the early 2000s.