'Agate' definitions:

Definition of 'agate'

noun

An impure form of quartz consisting of banded chalcedony; used as a gemstone and for making mortars and pestles

From: GCIDE

Chalcedony \Chal*ced"o*ny\ (k[a^]l*s[e^]d"[-o]*n[y^] or k[a^]l"s[-e]*d[-o]*n[y^]; 277), n.; pl. Chalcedonies (-n[i^]z). [ L. chalcedonius, fr. Gr. CHalkhdw`n Chalcedon, a town in Asia Minor, opposite to Byzantium: cf. calc['e]doine, OE. calcidoine, casidoyne. Cf. Cassidony.] (Min.) A cryptocrystalline, translucent variety of quartz, having usually a whitish color, and a luster nearly like wax. [Written also calcedony.] [1913 Webster]
Note: When chalcedony is variegated with with spots or figures, or arranged in differently colored layers, it is called agate; and if by reason of the thickness, color, and arrangement of the layers it is suitable for being carved into cameos, it is called onyx. Chrysoprase is green chalcedony; carnelian, a flesh red, and sard, a brownish red variety. [1913 Webster]

Agate (Heb. shebo), a precious stone in the breast-plate of the high priest (Ex. 28:19; 39:12), the second in the third row. This may be the agate properly so called, a semi-transparent crystallized quartz, probably brought from Sheba, whence its name. In Isa. 54:12 and Ezek. 27:16, this word is the rendering of the Hebrew cadcod, which means "ruddy," and denotes a variety of minutely crystalline silica more or less in bands of different tints.
This word is from the Greek name of a stone found in the river Achates in Sicily.