"Glittering jewels whose value increased after preserving the false deceased."
"The twins found a long, miniature table with a few dozen miniature chairs around it. It looked like an area where the dwarfs took breaks from working. A large portrait of Snow White hung on the dirt wall behind it, and a glass coffin with rubies and diamonds sat against the wall underneath it."
When they take a few, there are already other jewels missing from the coffin.
Apart from being an ingredient for the spell, it does not say if the jewels have any other magical qualities.