Question: what's the deepest rock/crystal that you have discovered?

Danny Hnatyshin answered on 16 Nov 2020:

So the deepest sample that I have ever collected to be used for my research were from oil well drills. They retrieved samples for my research were from about 3.5 km deep!
During university I did get to see samples that were once a few hundred km under the surface before being erupted in volcanoes! However, that is not very deep compared to the distance to the center of the earth which is about 6350 km from the surface.