yeah I think it's hardcoded for something like 10-17 above the bedrock base.
I just meant with real world diamonds most that can actually be mined are forced toward the surface by geological activity that actually create them.
"The Argyle Diamond Mine is the world's largest diamond mining operation by volume, although not necessarily by value. Located in Australia, this mine covers 110 acres, and reaches down underground about 1,900 feet (not even 1 Km!) at its deepest part."
and considering mining that's pretty shallow for that being the absolute deepest point, while with gold I think the record for depth is about 4.5 kilometers. 1Km being 3280.84 feet.