You also find lots of shiny ore so even that is not a big problem (already got 2 stacks of shiny ingots)
directly form wiki
Emerald ore is only generated in Extreme Hills biomes. Due to the nature of the generation algorithm, it may be found a short distance into adjacent biomes, but always within the same chunk as an Extreme Hills biome.
[1] It attempts to generate 3-8 times per each chunk, generating anywhere between layers 4 and 32, meaning it can occur as high up as
gold can.
In a world there are about 0.2 Emerald ores per chunk. It will only generate in locations which would have otherwise been stone - it is thus possible for as few as 0 to 2 emeralds to generate in a chunk. Found as individual blocks, rather than veins, it can be encountered more commonly than gold, which has a similar range.
also afew BoP biomes spawn it but i actually have no idea which are enabeld (havent found an Emerald hill biome yet (i think that's what it was called) found tons of emeralds in there in 1.6.4)