rock crystal 
noun [mass noun]
Transparent quartz, typically in the form of colourless hexagonal crystals: a piece of rock crystal [as modifier]: a rock crystal ball [count noun]: Scottish beachcombers can expect rock crystals and cairngorms...
- Crystal gazing seeks visions allegedly seen in a ball of rock crystal, preferably quartz.
- The rock crystal pieces were complemented by a positive kaleidoscope of coloured hardstones.
- However in the pagan period, rock crystal was used to make crystals balls about 2 inches in diameter.