Yes it is very good. I think if I were to write this I may add commas, but I am not sure it is necessary. The forgotten treasure, which has been discovered recently, is supposed to be worth a vast quantity of money.

There is some tautology here, which is why the sentence sounds awkward. If a treasure is recently discovered, it is likely to have been forgotten or unknown. However, even if that wasn't the case, the sentence sounds clunky and unclear. You are better off saying: 'The recently-discovered treasure is supposed to worth ....etc'

Yes. I agree with David. I might change the word 'quantity' to 'amount' though. Think it flows better. ;)