Diamonds and signed pieces are valued differently. A stone is judged on cut, colour, clarity and carat with the certificate if you have it; a signed piece from a house with secondary demand can be worth more than its metal and stones, because resale appetite is part of the value. We separate those two conversations so you can see which one your piece is in.
You do not need 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.