RocksdayRocksday #7 · October 28, 2026
The answer was Quartz
The second most abundant mineral in Earth's crust. Clear crystals grow as six-sided prisms capped by a pointed termination, and quartz shows up as grains in sand, granite and countless other rocks.
The clues
- Type
- Mineral
- Color & luster
- Clear, White, Gray · Vitreous
- Mohs hardness
- 7
- Crystal system
- Trigonal
- Found in
- Hot Springs, Arkansas; Minas Gerais, Brazil
How to recognize quartz
- Scratches glass easily and can't be scratched by a steel knife.
- Six-sided prisms with pointed ends when it forms crystals.
- Breaks with curved, shell-like (conchoidal) fractures — no flat cleavage planes.
- Does not fizz in vinegar or dilute acid.
Did you know? The quartz crystal in a watch vibrates exactly 32,768 times per second when electrified.