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

An uncut sapphire is an uncut gem that is used in the Crafting skill. Uncut sapphires can be cut at level 20 Crafting by using it with a chisel, yielding 50 experience. Sapphires can then be used to create sapphire jewelry.

Uncut sapphires can be obtained:

Shops

Store Location Stock
Al Kharid gem stall Al Kharid 1
Herquin's Gems Southwest Falador 1

Dropped by

All monsters with access to the rare drop table. Shadow Warriors have a relatively greater chance to use the rare drop table over other monsters.

Monster Combat level Quantity Rarity
Jailer 51 1* 7Unknown

* This drop may just be from the rare drop table and not an explicit drop from the monster.

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