A mineral form of silica WordNet 3.0 & Open English WordNet

Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Wikipedia

Silica mineral, polymorph of quartz Wikidata

A rare mineral of volcanic rocks that solidified at a high temperature, with the chemical composition silicon dioxide, SiO2. Tridymite is chemically identical to quartz, but has a different crystal structure. Sometimes contains sodium aluminum silicate, which seems to be expelled when quartz solidifies at lower temperatures. Wiktionary

Mineral. Wiktionary (translation)