Since the silicate minerals compose about 95 percent of the Earth’s crust and upper mantle, you might think they’re all rubbish. Well, look again and look closely.

Tridymite is a silica mineral (SiO2) not only valuable to collectors but also used in scientific study.

This rare and distinctive mineral’s crystals are usually microscopic; the largest are under 1 cm long and very thin. Here you see tridymite aggregate from Ettringer Bellerberg in Germany’s Eifel Mountains with an image width of only 2.5 mm.

Yet they provide key information on how crystals form and how they change in different environments. Tridymite is also synthetically produced to make refractory ceramics for linings in equipment such as hot stoves, open-hearth furnaces and blast furnaces used in smelting to produce industrial metals.

Tridymite is actually more common as a component of opal than as an individual mineral. The play of colours in opal arises from the diffraction of light from sub-microscopic layers of regularly oriented silica spheres.

Its unusual crystals and mode of occurrence distinguish tridymite from all other minerals.

Some of the finest examples are from the Euganean Hills in Italy. Other localities include Lyttelton Harbour in New Zealand, Cornwall in England, the Northern Slanske Mountains in Slovakia and Pachua in Mexico. In the US, tridymite occurs in the Thomas Range in Utah; the Obsidian Cliff in Yellowstone National Park, Wyoming; and Mt. Lassen in Lassen Volcanic National Park, California, among other places.

Tridymite is a high-temperature polymorph of quartz and usually occurs as minute tabular white or colourless pseudo-hexagonal crystals, or scales, in cavities in acidic volcanic rocks.  The mineral was first described in 1868 and given its name from the Greek tridymos for triplet, as tridymite commonly occurs as twinned crystal trillings.