vivianite crystal

"Vivianite Fe3(PO4)2·8(H2O), hydrated iron phosphate, is a secondary mineral found in a number of geological environments. Usually found as deep blue to deep bluish green prismatic to flattened crystals, most crystals rather small to microscopic, larger ones are rare.

"It is formed by the alteration of ore deposits near the surface, or of primary phosphates in pegmatites. Vivianite crystals are often found inside fossil shells, such as those of bivalves and gastropods, or attached to fossil bone.

"Vivianite darkens upon exposure to oxygen, and was named in 1817 after J.G.Vivian, an English mineralogist who first discovered crystals of the mineral in Cornwall." (from Wikipedia)