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This doesn't make sense. Diamonds are supposed to be one of the hardest material known.

Hardness is a surface property which is not the same as compressive strength. Something like water is not hard at all, but very resistant to compression.

You are an evil person to waste money this way...

There's nothing evil about this.* It seems they got the diamond for free, and the people who gave them it did so to advertise.

*If there is something evil about this, it's the racket of the diamond industry. They manipulate the market to artificially raise their price, engage in anti-competitive practices (especially against much cheaper and better synthetic diamonds), insidiously manipulate culture to make men believe they have to pay a lot of money for diamond engagement and wedding rings, and most evil of all: use child and slave labor to mine them.