AGATE is a 5 letter word that starts with A.

1. \A*gate"\, adv. [Pref. a- on + gate way.]
   On the way; agoing; as, to be agate; to set the bells agate.
   [Obs.] --Cotgrave.
   

2. \Ag"ate\, n. [F. agate, It. agata, L. achates, fr. Gr. ?.]
   1. (Min.) A semipellucid, uncrystallized variety of quartz,
      presenting various tints in the same specimen. Its colors
      are delicately arranged in stripes or bands, or blended in
      clouds.
   Note: The fortification agate, or Scotch pebble, the moss
         agate, the clouded agate, etc., are familiar varieties.
   2. (Print.) A kind of type, larger than pearl and smaller
      than nonpareil; in England called ruby.
   Note: This line is printed in the type called agate.
   3. A diminutive person; so called in allusion to the small
      figures cut in agate for rings and seals. [Obs.] --Shak.
   4. A tool used by gold-wire drawers, bookbinders, etc.; -- so
      called from the agate fixed in it for burnishing.
   

(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.