How could something that has no practical use be so expensive?
Because of it's hardness diamonds are used in drills and many other things. They also conduct heat efficiently. If you could make glasses out of them they would be thinner than glass ones and much more scratch resistant. There is even the possibility of very high temperature transistors made out of diamond for when you need control equipment in hot places.
They make an excellent abrasive.
And they make great anvils for experimental physics and materials science uses.
Cache...”look dear...some poor sap dug through the earth and found this rare example of a rare physical process. I paid money to have them clean it...and cut it, so its SPARKLES and gives you the willies.....and I love you”...
Isn’t that more romantic than...”Using cheap labtech labor, we superpressured hydrocarbon gas at very high temp and deposited the carbon layer by layer and shaped it to SPARKLE and give you the fake willies...and pass the sammich”?
So we add a tth C to diamond selection...cut, color, clarity, and carat... and now cache...