Here’s a chunk in my garden. See those horizontal cracks? They look almost like cleavage, but don’t all the mineral guides say quartz lacks cleavage?
Let me let you in on one of the dirty little secrets of geology. Some references do say quartz has cleavage, although it’s described as “weak.” The classic Introduction to the Rock Forming Minerals by W.A. Deer, R. A. Howie and J. Zussman says “A feeble cleavage may occur in quartz…” A possibly more accurate description is parting. Parting is a tendency of materials to break along crystal lattice planes, but it’s not universal. It’s related to impurities deposited along some crystal planes, or defects in the crystal structure on some planes but not all.
So anyway, if you get a piece of quartz and you see straight cracks here and there that look a bit like cleavage, it’s still quartz.
