From an unused root probably meaning to contain or sparkle; perhaps a pocket (as holding the Urim and Thummim), or rich (as containing gems), used only of the gorget of the highpriest -- breastplate.
ח֫שֶׁן noun masculine the breast-piece or sacred pouch, containing the ותמים אורים, worn on the breast of the high priest when he ministered in the Holy Place to bring the tribes for memorial. before ׳י. It was made of the same material as the ephod (see אֵפוֺד), a span square; set in front with twelve jewels in four rows, engraved with the names of the twelve tribes. It was firmly fastened to the shoulders of the ephod by gold chains passing through gold rings, and to the lower part of the ephod just above the girdle by a blue ribbon passing through other gold rings. It was (ה)מִשְׁמָּט חשֶׁן ( Exodus 28:15, 29, 30) pouch of judgment, because of the decision given by the וְתֻמִּים אוּרִים. The name occurs only in P, 25:7; 28:4, 15, 22, 23 (twice in verse); 28:24, 26, 28 (twice in verse); 28:29-30, 29:5; 35:9, 27; 39:8-9, 15, 16, 17, 19, 21 (twice in verse); Leviticus 8:8 (twice in verse). — See NowArchaeology ii. 119.