Množina: agates

ETYM French agate, Italian agata, Latin achates, from Greek.
An impure form of quartz consisting of banded chalcedony; used as a gemstone and for making mortars and pestles.
Banded or cloudy type of chalcedony, a silica, SiO2, that forms in rock cavities. Agates are used as ornamental stones and for art objects.
Agate stones, being hard, are also used to burnish and polish gold applied to glass and ceramics.