RocksdayPlay

Rocksday #8 · October 29, 2026

The answer was Quartzite

Sandstone fused by metamorphism into an extremely hard rock of interlocked quartz grains.

Quartzite — the Rocksday #8 puzzle photo
The photo from that day's puzzle, via Wikimedia Commons — credits

The clues

Type
Metamorphic rock
Color & luster
White, Pink, Gray · Sugary to glassy
Mohs hardness
7
Texture
Interlocking quartz grains (non-foliated)
Found in
Baraboo, Wisconsin; Sioux Falls, South Dakota

How to recognize quartzite

  • Breaks through the grains, not around them.
  • Scratches glass.
  • Smooth, sugary surface — not gritty.
  • Doesn't fizz in acid.

Did you know? The pink Sioux Quartzite is so durable it was used to build much of Sioux Falls.

Easy to confuse with

Full Quartzite guidePlay #8Today's puzzle