Diamond

Diamond is the material a mid-game world is measured in, and most questions about it are really questions about depth. Diamond ore generates from roughly Y=16 all the way down to the bedrock floor and grows steadily more common the deeper you go, which is why the standard answer to the diamond level question is to mine near the bottom: around Y=-59 the odds are as good as they get, and below that is bedrock.

Two details decide how many you actually walk away with. An iron pickaxe or better is required — anything weaker breaks the ore and leaves nothing behind — and a large share of diamond ore simply does not generate where it would have been exposed to open air, so ore sealed inside stone is more common than ore visible in a cave wall. That is why a branch mine at depth tends to out-earn cave diving, and why Fortune III on the pickaxe is worth more than any amount of extra tunnel.

What diamonds buy is reach. A diamond pickaxe is what mines obsidian and ancient debris, and every piece of netherite gear begins as a diamond one that you upgrade at a smithing table rather than craft from scratch — so the diamond sword or diamond armor you enchant now is the same gear you will still be carrying much later, enchantments included.

Nine of them press into a block of diamond, which is both the compact way to store a haul and one of the materials a beacon pyramid accepts. They also appear in chest loot across the world, so the quickest route to a first pickaxe is often a shipwreck, a buried treasure or a village blacksmith rather than a mine at all.