Yep.  and 90% of the world's diamonds are in vaults now and they trickle out a supply every so often to keep the demand up.

The real way to tell the worth of a gemstone is what it sells for.  A Ruby, for instance, will sell for just about what you bought it for most of the time.  Same with a Sapphire.(technically they are the same other than color - interesting factoid) Same with Silver and Gold as well.  But Diamonds are simply off - they sell for 3-4X more than you can get back from them.

Plus, they are honestly, ugly.  It's like the white Toyota Camry of the gemstone world.   Bland, soulless, and everyone has one.