Hello,
the source is the US Mine Safety & Health Administration (ref:US:MSHA, https://www.msha.gov/data-and-reports/mine-data-retrieval-system - government data, loads very slowly but is at least public domain)

I invented the rock_type tag because well; I want to express weather the mined dimension rock is granite or marble. I also invented aggregate:rock_type, to express which rock type the aggregate consists of - granite, limestone or something else. ("aggregate" is probably better known as "crushed rock", used for concrete, asphalt and other construction)
I also invented "ref:US:MSHA" because the MSHA records contain further valuable information, such as operator history, accidents, safety violations & more stuff. (Key's format based at similar "ref:US:EIA" for US power plants)

Hope this answers your questions!
Greetings