So called, says Pliny (xxxvii. 10), from Achates or Gagates, a river in Sicily, near which it is found in abundance.
These, these are they, if we consider well, That saphirs and the diamonds doe excell, The pearle, the emerauld, and the turkesse bleu, The sanguine corrall, amber's golden hiew, The christall, jacinth, achate, ruby red.
Agate is supposed to render a person invisible, and to turn the sword of foes against themselves.
A very diminutive person. Shakespeare speaks of Queen Mab as no bigger than an agate-stone on the forefinger of an alderman.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894