Albert Found a Stone in His Yard and Took It

Albert found a stone in his yard and took it to Bob, a jeweler, for evaluation. Bob wasn't sure as to the nature of the stone, but told Albert he thought it was a topaz. Bob then offered to buy the stone for $25 and Albert agreed. Later Albert found out the stone was an uncut diamond worth about $700. The sales contract:

A) is a valid contract that should be enforced by the law, because neither party knew the exact nature of the stone at the time of the sale.
B) can be voided based upon fraud in the execution.
C) can be voided based upon fraudulent misrepresentation.
D) can be voided based upon mistake as to the identity of the subject matter.