Geology

Opal is hydrated silica (SiO₂·nH₂O), a mineraloid with no crystal structure that can hold up to about 20% water. It rates 5.5–6 on the Mohs …

Petrified wood is a fossil in which the original wood has been replaced by minerals, mostly quartz, while keeping its rings and grain. Silica-rich pieces …

A geode is a hollow, roughly spherical rock – usually formed from a gas bubble in volcanic rock or a cavity in sedimentary limestone – …

Agate is a banded variety of chalcedony (microcrystalline quartz), built up in fine layers inside gas pockets in volcanic and metamorphic rock, and it typically …

Moonstone is a variety of the feldspar minerals orthoclase and albite, prized for adularescence – the floating, moonlight-like sheen created by microscopic layers of the …

Everything you need to know about emerald – how it forms underground, how to spot a real one, what drives its price, and where to find it in the U.S.