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Cyclopedia of Biblical, Theological and Ecclesiastical Literature
(שְׁבוֹ , shebo', signif. unknown; Sept. ἀχάτης, Vulg. achates), a precious, or rather ornamental stone, which was one of those in the breastplate (see Braunii Vest. Sacerd. Hebrews 2:15) of the high-priest (Exodus 28:19; Exodus 39:12). The word agate, indeed, occurs also in Isaiah 54:12, and Ezekiel 27:6, in our translation; but in the original the word is כִּדְכֹּד, kadkod. See RUBY. Theophrastus describes the agate as "an elegant stone, which took its name from the river Achates (now the Drillo, in the Val di Noto), in Sicily, and was sold at a great price" (58). But it must have been known long before in the East, and, in fact, there are few countries in which agates of some quality or other are not produced. The finest are those of India; they are plentiful, and sometimes fine, in Italy, Spain, and Germany. We have no evidence that agates were found in Palestine. Those used in the desert were doubtless brought from Egypt. Pliny says that those found in the neighborhood of Thebes were usually red veined with white. He adds that these, as well as most other agates, were deemed to be effectual against scorpions, and gives some curious accounts of the pictorial delineations which the variegations of agates occasionally assumed. Agate is one of the numerous modifications of form under which silica presents itself, almost in a state of purity, forming 98 per cent. of the entire mineral. The silicious particles are not so arranged as to produce the transparency of rock crystal, but a semi-pellucid, sometimes almost opaque substance, with a resinous or waxy fracture, and the various shades of color arise from minute quantities of iron. The same stone sometimes contains parts of different degrees of translucency, and of various shades of color; and the endless combinations of these produce the beautiful and singular internal forms, from which, together with the high polish they are capable of receiving, agates acquire their value as precious stones. Agates are usually found in detached rounded nodules in that variety of the trap rocks called amygdaloid or mandelstein, and occasionally in other rocks. Some of the most marvellous specimens on record were probably merely fancied, and possibly some were the work of art, as it is known that agates may be artificially stained. From Pliny we learn that in his time agates were less valued than they had been in more ancient times (Hist. Nat. 37, 10). The varieties of the agate are numerous, and are now, as in the time of Pliny, arranged according to the color of their ground. The Scripture text shows the early use of this stone for engraving; and several antique agates, engraved with exquisite beauty, are still preserved in the cabinets of the curious. (For a further account of the modern agate, see the Penny Cyclopcedia, s.v.). (See GEM).
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McClintock, John. Strong, James. Entry for 'Agate'. Cyclopedia of Biblical, Theological and Ecclesiastical Literature. https://www.studylight.org/encyclopedias/eng/tce/a/agate.html. Harper & Brothers. New York. 1870.