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Rare Blue Diamonds Lurk Deep In Earth's Core (washingtonpost.com)
Scientist believe they now know how extremely rare blue diamonds are formed. After studying 46 of the gems, they found that blue diamonds are formed as deep as the transition zone between the planet's upper and lower mantle (Warning: source may be paywalled; alternative source) -- far greater depths than other diamonds, some deeper than 410 miles. The Washington Post reports: Just 1 out of 200,000 diamonds are blue. Like all diamonds, they are made when carbon comes under intense pressure and extreme heat deep inside the Earth. As they form, they can trap tiny bits of rock inside -- like fossils in amber. "Diamond is an extraordinary container, a time capsule," said Steven Shirey, a geochemist at the Carnegie Institution for Science in Washington. Shirey and his colleagues used lasers to examine the diamonds' imperfections -- slivers of embedded rock -- at the Gemological Institute of America. The researchers suggest that boron in the ocean floor was pushed down when plates that make up the Earth's crust collided. The element allows the stone to absorb some red light, so the diamond looks blue. The findings were published in the journal Nature.
... is exactly why humans values these stones so highly. Sure, they're hard, but thats a pretty niche use. Apparently they look pretty. To me they just look like bits of glass which also doesn't tarnish. I honestly don't get why our species craves this stone so much.
The deepest mine in the world is about 4km underground...
The main obstacles to going deeper are getting air in and getting heat out. The solution is to get humans out and robots in. Robots don't need oxygen, and they can tolerate much higher temperatures. They can also work in much narrower shafts, and can even work submerged in pressurized liquids that can reduce cave-ins. There are also huge savings from avoiding all the safety measures, shift changes, and even gem theft.