钅 is the left-side form of 金 (metal). It marks 镶 as a metalworking action - setting jewels, riveting borders, fitting metal trim - and links it to 银, 铜, 钢, 锁 and the rest of the smith's vocabulary.
襄 supplies the sound xiāng directly. It also carries a faint semantic echo of wrapping or enveloping, which fits 镶 nicely: setting a stone is wrapping it in a metal frame. Same phonetic in 让 (ràng), 嚷 (rǎng), 壤 (rǎng).
