RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Uncut Opal
The Old School RuneScape Wiki also has an article on Uncut Opal.

An Uncut Opal is a semi-precious gem that is used in the Crafting skill. It can be cut at level 1 Crafting by using it with a chisel, yielding 15 experience and an opal if successful. There is a chance to fail when cutting the gem, resulting in a useless crushed gemstone for 3.75 exp.

Uncut opals can be obtained either by panning at the Digsite or Mining gem rocks in northwest Shilo Village.

Besides cutting the gems for experience, the only use opal has in RuneScape Classic is during the Legend's Quest, where one must be placed on the correct Carved Rock within Legends caverns.