rock crystal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Colorless transparent quartz, used in optical instruments and as a semiprecious gemstone.

from The Century Dictionary.

• noun See crystal, and cut under pokal.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun clear, colourless form of the silica-based mineral quartz, often called "pure quartz" or "clear quartz".

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a clear quartz used in making electronic and optical equipment