Emerald Ore
Emerald Ore is a rare mineral block found in individual blocks scattered about the depths of Extreme Hills biomes at y levels between 4 and 32 (beginning about halfway through the second strata). It can be chipped by an Iron Chisel to produce a single Emerald, or the entire block can be destroyed with an Iron Pick or above for to produce an Emerald and six Loose Stones.
Notice that an average extreme hills chunk has fewer emeralds than even diamonds. This together with the danger presented by silverfish makes mining Emerald Ore a very unattractive method of gathering Emeralds in comparison to trading.
Unlike metal ores, Emerald Ore collected through Silk Touch cannot be fired in a Kiln—it must be placed in the world and mined again without Silk Touch. This is recommended anyways, because the ore is affected by the Fortune enchantment; Every level of Fortune increases the max yield of emeralds by one, so that a Fortune III pick will yield 1–4 emeralds per ore block.