Tridymite

Tridymite, a mineral composed of silica, which occurs in various acid igneous rocks in the form of thin transparent six-sided plates, several of which are usually grouped together. It is very brittle, has a hardness of 7, and specific gravity of 2.25 to 2.33. The mineral occurs in cavities in the trachytes of many regions, as in those of the Siebengebirge, Mont Dore, Hungary, Euganean Hills, Ireland, Iceland, Mexico, &c. It is also met with scattered through the ground-mass of trachytic rocks, as at Kosemütz in Silesia, Kaschau in Hungary, Hüttenberg in Carinthia, &c.