What is the rarest ore in Minecraft?

What is the rarest ore in Minecraft?

Emerald OreEmerald Ore is the rarest ore to date in Minecraft (25 times more so than Diamond Ore, while almost always spawning in only one-block veins). When mined (with an Iron Pickaxe or higher), it drops an Emerald; the only implemented use of Emeralds to date is for trading with Villagers.