Diamonds don't have resale value anyway. They're literally a scam.
As for gold jewelry, this can swing both ways. An intricate piece with some sort of history attached may fetch a higher price from a buyer that is interested in that versus a scrap buyer that will just melt it down.
Similar analogies can be found with cars. A factory stock 1996 VW Golf will only be worth slightly higher than scrap value. Meanwhile, a 1996 VW Golf Harlequin will be worth $8-9k, its the same car but with a different factory paintjob. Effectively no difference in manufacturing costs since it was just made from different stock-painted panels but worth way more to a buyer simply due to the history and rarity. The cars both still have the same scrap value, a junkyard would buy both of them for the same price.
I've found aluminum globs under fires that had cans or trashed roofing thrown in. Pretty clean, since nearly everything floats on it, and easy to spot in ashes.
(Lead, on the other hand, melts at half that.)
The fires I have found aluminum globs in were long burning beach campfires and a bonfire to burn brush(big fire). The beach campfires were moderately sized but they certainly weren't bonfires built with 6-8 foot logs.