RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Pestle and mortar
The Old School RuneScape Wiki also has an article on Pestle and mortar.

A Pestle and mortar is used to grind items to dust, which are then used in quests or as secondary ingredients for making potions.

Item Becomes Uses
Bat bones
Bat bones
Ground bat bones
Ground bat bones
Ogre potion during the Watchtower quest
Blue dragon scale
Blue dragon scale
Ground blue dragon scale
Ground blue dragon scale
Weapon poison
Chocolate Bar
Chocolate bar
Chocolate dust
Chocolate dust
Hangover cure during the Plague City quest, Cooking chocolate cake, & some Gnome food/drink
A lump of Charcoal
A lump of charcoal
Ground charcoal
Ground charcoal
Explosive compound during the Digsite quest
Unicorn horn
Unicorn horn
Ground unicorn horn
Ground unicorn horn
Cure poison & Poison antidote potions

Stores[]

Pestle and mortar costs 4gp in Herblaw shops and 6gp in southern Karamja General Stores.

Store Location Stock
Entrana Herblaw Shop Entrana 3
Obli's General Store Shilo Village 3
Jungle Store Tai Bwo Wannai 3
Jatix's Herblaw Shop Taverley 3