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 one of the rarest materials in Minecraft.

Obtaining
One word:

DIG.

Mining
Diamond ore mined using an iron or diamond pickaxe will drop a single diamond. 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.

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

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

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

Advancements
Easter Egg: The 'Cover Me With Diamonds' advancement is coded as 'shiny_gear'.

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.