People have a lot of funny misconceptions about diamonds. Diamonds are pretty much at the top of the scale with respect to hardness. What that means is that if you scratch pretty much anything against a diamond, the diamond will win. The other thing will get scratched and the diamond won’t.
Diamonds are very hard, but they aren’t tough. Whack a diamond with a hammer and you’ll easily shatter it. You have to be careful with diamond rings and other diamond jewelry because just accidentally smacking the diamond on something can fracture it and ruin it.
I’ve noticed that a lot of folks confuse hardness and toughness, and end up thinking that diamonds are virtually indestructible. They aren’t. Not even close.
If the shock wave from the nuke didn’t blast the diamond into smithereens, diamonds have another property that folks don’t seem to be aware of much. They burn. They don’t burn so easily that you have to worry about catching them on fire with a candle, but you can burn one with a blowtorch. The heat from a nuke exceeds that by quite a bit, so I’d expect all of your tiny diamond fragments (from it being blasted apart by the shock wave) to catch fire and burn away into nothing (well, technically, carbon dioxide).