Leucite — KAlSi₂O₆, a feldspathoid of potassium-rich, silica-poor lavas (Vesuvius). Its "cubic" crystals go tetragonal on cooling.

White–grey, vitreous, Mohs 5.5–6.
Spin below.

Specimen image: AI-generated.
https://apps.apple.com/us/app/rockspin/id6747876962
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