If you use a chisel on a cut opal, you will receive 12 opal bolt tips that can be added to regular bronze crossbow bolts. Alternatively, an opal can be sold to Gabooty in Tai Bwo Wannai Village for trading sticks.
Notes:
Uncut opals can be mined at the Shilo Village Gem Mine, at rocks randomly spawned during the Tai Bwo Wannai Cleanup Minigame, or panned at the Digsite river.