The story
The name refers to the peculiar habit of crystallizing in groups of three individuals.
Etymology from Wiktionary, CC BY-SA 4.0.
tridymite
Today it means: 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. Full definition →