Minecraft wiki entry for Coal_Ore, associated text: Coal ore can generate in the Overworld in the form of blobs. It generates in two batches per chunk. The first batch attempts to generate 30 times per chunk in blobs of 0-37, evenly from levels 136 to 320. The second batch attempts to generate 20 times per chunk in blobs of 0-37, from levels 0 to 190, being most common at Y=95, and being less common as towards either end of the range. Coal ore in this second batch is also less likely to be exposed to air; 50% of ore blocks that would generate exposed do not generate. In Bedrock Edition, coal ore also attempts to generate 20 times per chunk in blobs of size 0-37, evenly from levels 128 to 256, within any type of mountains biome.