quartz crystal

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A small crystal of quartz accurately cut along certain axes so that it can be vibrated at a particular frequency, used for its piezoelectric properties to produce an electric signal of constant known frequency.

WordNet 3.0

1. n. a thin plate or small rod of quartz cut along certain lines and ground so that it can produce an electric signal at a constant frequency; used in crystal oscillators