Diamonds and signed pieces get valued differently. A stone is judged on cut, colour, clarity and carat, with the certificate if you have one. A signed piece from a house with resale demand can be worth more than its metal and stones, because appetite for it is part of the value. We keep those two conversations separate so you can see which one your piece is in.
You do not have to sell. If the piece has family meaning, a loan keeps it yours. It sits in the vault, you redeem it, and it never changes hands. Ask for the redemption date and the interest in writing before you sign.