Diamond

"Diamonds are a miner’s best friend, a blue gleam in the darkness that you can use to craft the best stuff. The best stuff. Pickaxes that can hack through obsidian, swords that can hack through wither skeletons, armour that can withstand the force of 1000 spiders. Diamond is the best! Or is it?"

- Alex Wiltshire

Diamonds are a rare mineral obtained from diamond ore or loot chests. They are mainly used to craft the highest tier of tools and armor, and the enchantment table.

Mining
Diamond ore mined using an iron or diamond pickaxe will drop a single diamond. If mined by any other tool, it will drop nothing. If the pickaxe is enchanted with Fortune, it can drop an extra diamond per level of Fortune, allowing for a maximum of 4 with Fortune III. If the ore is mined using a pickaxe enchanted with Silk Touch, it will drop an ore block rather than a diamond. The ore is found at level 16 or lower, most commonly between 5 and 12.

Usage
Diamonds are used to craft the most durable tier of tools and armor.

Trading
Villager armorers, toolsmiths and weaponsmiths buy 3-4 diamonds for one emerald as part of their third tier trades.

Expert-level armorer, toolsmith, and weaponsmith villagers buy one diamond for an emerald as part of their trades.

Journeyman-level Armorer villagers have a 40% chance of offering to buy one diamond for one emerald. Expert-level Toolsmith villagers have $2/3$ chance of offering to buy one diamond for one emerald. Expert-level Weaponsmith villagers will always buy one diamond for one emerald.

Repairing
Diamond helmets, chestplates, leggings, boots, swords, pickaxes, axes, hoes and shovels can be repaired with diamonds in an anvil.

Beacons
Diamonds can be used to select powers from a beacon. The player must select one of the available powers then insert a diamond into the item slot.

A diamond can be substituted for an iron ingot, a gold ingot, or an emerald.

Trivia

 * In real life, diamond is commonly used for industrial purposes, generally as diamond-tipped drill bits and saw blades. They are usually used for cutting stone and other hard materials. Diamonds used for this purpose are typically not of gemstone quality.
 * Diamonds are technically renewable in Legacy Console Edition because of the ability to reset the Nether and the End, as diamonds generate in fortress and end city chests.