Jewelcrafting

Jewelcrafting is an extremely valuable profession introduced in The Burning Crusade in which a player can learn to cut hundreds of different stat-boosting gems, which can be socketed into player equipment. A jewelcrafter can use Prospecting on ore gathered via Mining to extract gems of various colors. An expert jewelcrafter must complete daily quests for currency required to purchase the highest quality epic gem cut recipes, although some recipes are world drops or from reputation vendors.

The type of ore determines the quality of the gem prospect. In Cataclysm, Obsidium Ore has a low chance to yield rare gems, Elementium Ore has a higher chance, with Pyrite Ore having the greatest chance at rare gems and a shot at Volatile Earth too.

Jewelcrafters can cut uncommon, perfect, rare, and epic gems for different amounts of a stat, depending on the quality of the gem. Gem Perfection, formerly a reward for completing a Northrend quest, is now a passive component of Jewelcrafting. In addition, necklaces and rings that require 450+ Jewelcrafting can proc perfect versions too.

In addition to cutting gems, jewelcrafters can also craft rings, necklaces, and weapons. These items are great for leveling and also yield a decent quantity of Enchanting materials.

In Wrath, players could turn uncommon gems into rare and epic gems once a day with Icy Prism. Epic gems are not implemented in Cataclysm yet, but there's something similar: Fire Prism.

Jewelcrafters can make some fun items. There's a few vanity monocles from BoE designs. In Wrath, players learned how to craft several Focusing Lens which created a neon beam between the player and the target. Even at low levels, there's quirky stuff like Rough Stone Statue.

If you cut gems in a guild, it will count towards Gemcrafter Extraordinaire.
Profession-specific Perk
Jewelcrafters can cut Chimera's Eye into special gems with increased stats that only they can equip, though they are limited to equipping only three of these gems at a time.

Jewelcrafters have always had access to BoP trinkets. In Cataclysm, jewelcrafters randomly loot items that begin quests, yielding new trinkets