Rashi on Terumah 25:7

English: אבני שהם ONYX STONES — two were required there for the needs of the “ephod” which is mentioned in ואתה תצוה (Exodus 28:6ff.). מלאים FOR SETTING (lit., filling in) — Because they made for them (for the stones) settings in gold — a kind of indentation — and they put the stone there (in it) to fill the indentation, they were called “filling-up stones”; the spot where the indentation is (i. e. the hollow spot) is called משבצת “setting”. לאפד ולחשן FOR THE EPHOD AND THE BREASTPLATE respectively: the onyx stones for the ephod and “stones for setting” for the breastplate. Ephod and breastplate are described in ואתה תצוה. They were a kind of ornament worn only by the High-Priest.