Vivianite

Vivianite is a hydrated iron phosphate with one of the most dramatic color-change behaviors in mineralogy. Freshly mined vivianite is often colorless or pale green. Upon exposure to light, the iron in its structure oxidizes from Fe²⁺ to Fe³⁺, and the crystal turns progressively deeper blue - from pale blue-green to deep indigo to near-black over time. This color change is irreversible.

This photosensitivity means that the most deeply colored vivianite specimens are also the oldest in terms of light exposure. Museum specimens collected decades ago may be nearly opaque dark indigo. Collectors who want to preserve pale color must store vivianite in complete darkness.

Vivianite forms in reducing (oxygen-poor) environments - waterlogged soils, bogs, and fossils. It's frequently found inside fossil bones and shells where phosphorus from the original organism combines with iron in the surrounding sediment. Finding vivianite growing inside a dinosaur bone or a clam fossil is genuinely one of the more remarkable intersections of biology and geology.

Vivianite is identified by its extreme softness (hardness 2, scratched by a fingernail), perfect cleavage in one direction producing thin, flexible crystal blades, and the diagnostic darkening with light exposure. Fresh vivianite may be nearly colorless; older specimens are deep blue.

Distinguish from lazulite (much harder at 5.5, doesn't darken), azurite (harder, copper-based not iron), and erythrite (cobalt arsenate, pink-red). Vivianite's extreme softness, perfect cleavage, and photosensitive darkening are essentially unique.