The second batch attempts to generate 20 times per chunk in blobs of 0-37, from levels 0 to 192, being most common at Y=96, and being less common as towards either end of the range. The first batch attempts to generate 30 times per chunk in blobs of 0-37, evenly from levels 136 to 320. Coal ore can generate in the Overworld in the form of ore blobs.
