Verses of Exodus 28:13-14 speak of special golden mountings that were to be attached to the two stones on the Ephod (Vestment). Let's take a look:
12 וְשַׂמְתָּ֞ אֶת־שְׁתֵּ֣י הָאֲבָנִ֗ים עַ֚ל כִּתְפֹ֣ת הָֽאֵפֹ֔ד אַבְנֵ֥י זִכָּרֹ֖ן לִבְנֵ֣י יִשְׂרָאֵ֑ל וְנָשָׂא֩ אַהֲרֹ֨ן אֶת־שְׁמוֹתָ֜ם לִפְנֵ֧י יְהוָ֛ה עַל־שְׁתֵּ֥י כְתֵפָ֖יו לְזִכָּרֹֽן׃ ס
13 וְעָשִׂ֥יתָ מִשְׁבְּצֹ֖ת זָהָֽב׃
13 And thou shalt make ouches of gold;
14 And two chains of pure gold at the ends; of wreathen work shalt thou make them, and fasten the wreathen chains to the ouches.
Here are our two "onyx" stones set in their golden fillings:
And here are two golden mountings that could be attached to the golden fillings of the stones:
As you can see from this next rendering, these golden mountings (ouches) that the Exodus 28:13 speaks of were most likely in a form of flat hooks that could be "clamped" in a way to the golden fillings of the two stones of the Ephod:
Next, in Exodus 28:14, the description states that there were two golden chains attached to the ends of these two golden mountings. Like so:
As you can see, now we have two golden mountings with two golden chains attached to them. As you can also see, these mountings with chains were completely removable from off the Ephod stones.
And here is the final rendering of both mountings with chains on top of the Ephod. These two mountings with chains will play very important role in helping to secure Ephod's parts, as it will become apparent from the following verses.