Vivianite is a hydrated iron phosphate mineral that is usually a bottle green color. It is, however, very sensitive to light, and will darken to near black if left exposed. This is caused by the iron components within the crystal system changing their oxidative state. For this reason, specimens should be stored in the dark. The crystals themselves are usually flattened, but can also be fibrous, needle-like, prismatic, or massive — thin examples are flexible to a degree. Vivianite is often associated with the minerals siderite, sphalerite, quartz and various others, and is sometimes found on or inside fossils such as shells or bones. It was first discovered in Cornwall, England in 1817.