OnyX is a popular freeware utility for Mac OS X developed by French developer Joël Barrière that is compatible with both PowerPC and Intel processors. As a multifunctional tool for maintenance and optimization, it can control many basic Unix programs already built into OS X, including setting hidden preferences otherwise modified by using property list editors and the command line.

Onyx is a banded variety of the oxide mineral chalcedony. Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands: agate has curved bands and onyx has parallel bands. The colors of its bands range from white to almost every color (save some shades, such as purple or blue). Commonly, specimens of onyx contain bands of black and/or white.