I understand that diamonds are the kind of Veblen goods for which people are willing to spend a lot of money because they are expensive, in a logic-defying circular way, and that the presence of a cartels- makes prices artificially high, but there are still two things that I can never wrap my head around:

1) That if someone wants to sell a used diamond, they get pennies on the dollar. It's reasonable that they would sell at a small discount compared to wholesale prices, because of the extra work of authenticating it, the small volume etc., but the difference is so great that one assumes that some businesses would step in and take advantage of the arbitrage opportunity

2) That there is any difference at all in prices between mined and lab-grown diamonds. They are literally the same material, and lab-grown diamonds are as "real"as the mined ones

Just consider gold. You can sell used gold at a small discount compared to spot prices, and if there was a cheap way to make "lab grown" gold, I am sure it would sell at exactly the same price as mined gold