Agate is a microcrystalline variety of quartz, chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks but can be common in certain metamorphic rocks.
In middle earth Chalcedony,white or grey banded agates, and the Moss agate, agates with moss like dark green patterns, were known.
- MERP Rulebook 2nd Edition
- The Gems of Middle-earth, online essay by William E Wilson