Obsidian charms may be obtained as a drop from any TzHaar monster, though the drop rate is increased with the better monsters. The charms are also secondary drops, so you may still get obsidian weapons and armour or Onyx bolt tips along with the charms. They are used to summon the Spirit Tz-Kih, the Obsidian golem, and the Lava titan using the Summoningskill.
These charms are unstackable and untradeable. Like the abyssal charms, which also are unstackable, it is used as a tertiary ingredient in making Summoning pouches. The triple charm bonus from Familiarisation increases the quantity of Obsidian Charms gained per drop.
Familiars that can be made with the obsidian charm include: