Concise Bible Dictionary:

1. kadkod, ἴασπις: from the root to strike fire, and hence a “sparkling gem, ruby” (Gesenius). “A shining, sparkling precious stone, probably a carbuncle, so called from its red sparkling color” (Furst). It occurs but twice: Isaiah. 54:12, “I will make thy windows of agates.” This might be thought to signify that the agate was transparent, and that the windows were to be glazed with agates; but windows were not then glazed. Others translate the word minarets, pinnacles, battlements, and instead of agates many prefer rubies. The other passage is Ezekiel 27:16: “Syria was thy merchant for coral, the agate”; they came from Damascus. The Revised Version and others again translate “rubies” which well agrees with the signification of the word.
Carbuncle Garnet
2. shebo, ἀχάτης, achates, Vulgate, from to burn, to glow (Fürst) . “A composite stone formed of quartz, chalcedony, cornelian, flint, jasper, and therefore glittering with different colors” (Delitzsch). Bochart traces the word to a root signifying “to be spotted.” This gem has been described as a semipellucid, uncrystallized variety of quartz; it is found in parallel or concentric layers of various colors, and presents different tints in the same specimen. It is generally translated “agate.” It was the stone chosen for the second place in the third row of the breastplate of the high priest (Ex. 28:1919And the third row a ligure, an agate, and an amethyst. (Exodus 28:19); Ex. 39:1212And the third row, a ligure, an agate, and an amethyst. (Exodus 39:12)).
Chalcedony (var. Agate)

Strong’s Dictionary of Hebrew Words:

from an unused root (probably identical with that of 7617 through the idea of subdivision into flashes or streamers (compare 7632) meaning to flame; a gem (from its sparkle), probably the agate
KJV Usage: