Boyd’s Bible Dictionary:

The breastplate of the high priest (Ex. 28:15), was of embroidered stuff, some 10 inches square; its upper corners fastened with gold or lace to the ephod, its lower to the girdle (Ex. 28:28). Adorned with 12 precious stones (Ex. 28:12-29).

Concise Bible Dictionary:

Armor for the breast.
1. Of righteousness (for Christ) (Isa. 59:17); (for the Christian’s conflict in the heavenlies) (Eph. 6:14).
2. Of faith and love (for the wilderness) (1 Thess. 5:8).
3. Of iron (steeled conscience) (Rev. 9:9).
4. Of fire, jacinth, and brimstone (dire judgments) (Rev. 9:17). See ARMOR.

From Manners and Customs of the Bible:

1 Samuel 17:5. He had an helmet of brass upon his head, and he was armed with a coat of mail.
1. In the earliest times helmets were made of osier or rushes, and were in the form of bee-hives or skull-caps. The skins of the heads of animals were sometimes used. Various other materials were employed at different times. The ancient Egyptian helmet was usually made of linen cloth quilted. It was thick and well padded, sometimes coming down to the shoulder, and sometimes only a little below the ear. The cloth used was colored green, or red, or black. The helmet had no crest, but the summit was an obtuse point ornamented with two tassels. The Assyrian helmet was a cap of iron terminating above in a point, and sometimes furnished with flaps, covered with metal scales and protecting the neck. The Philistine helmet, as represented on ancient monuments, was of unique form. From the head-band there arose curved lines, by which the outline of the helmet was hollowed on the sides and rounded on top.
Goliath’s helmet was doubtless of this shape, and, being made of brass, must have presented a beautiful appearance. The form of the Hebrew helmets is unknown; but they probably did not vary widely from the Egyptian. As is seen in verse 38 they were sometimes made of brass. The helmet is also mentioned in 2 Chronicles 26:14; Jeremiah 46:4; Ezekiel 23:24; 27:10; 38:5.
2. For the body, the skins of bents were probably the earliest protection in battle. Felt or quilted linen was also used subsequently. The ancient Egyptians had horizontal rows of metal plates well secured by brass pins. The ancient Assyrians had scales of iron fastened on felt or linen. Iron rings closely locked together were likewise used by different nations. Scales made of small pieces of horn or hoof were also used. Sometimes a very serviceable armor was made of small plates of metal, each having a button and a slit, fitting into the corresponding slit and button of the plate next to it. It is supposed that Ahab had on armor of this sort when he was slain; the “joints of the harness” being the grooves or slits in the metallic plates, or the place between, where they did not overlap (1 Kings 22:34; 2 Chron. 18:33). Goliath’s “coat of mail” was scale armor (shiryon kaskassim, “armor of scales”). This kind of armor consisted of metallic scales rounded at the bottom and squared at the top, and sewed on linen or felt. The Philistine corselet covered the chest only. On the bas-relief at Nineveh are seen warriors with coats of scale armor which descend to the knees or ankles. In one of the palaces Mr. Layard discovered a number of the scales used for this armor. Each scale was of iron two to three inches long, rounded at one end and squared at the other, with a raised or embossed line in the center, and some were inlaid with copper. At a later period the Assyrian armor was made of smaller scales, which were pointed and ornamented with raised figures, and the coat of mail reached no lower than the waist.
In several passages shiryon is rendered in our version “habergeon.” See 2 Chronicles 26:14; Nehemiah 4:16.
The lorica of the Romans end the thorax of the Greeks—rendered “breastplate” in Ephesians 6:14 and 1 Thessalonians 5:8—were scale armor covering breast and back.