Sapphire

Sapphire is a gem added by various mods. Their main purpose is to craft tools.

Sapphire tools are the tier between Iron and Diamond. While they have the speed of diamond, they only have 501 uses, and can't mine Obsidian.

Sapphires can occasionally be found in Dungeon chests.

Engineer's Toolbox
Sapphires are currently not used for anything, but they will be used in the future.