Answer: The Jeweler
Explanation:
The law states that children are to be considered legally incompetent and as such can't effectively protect their own interests. Since they are legally incompetent to protect their interests, this responsibility now falls on the individual they are dealing with.
The Jeweler is expected to protect the rights of the young girl and as where it is proven that her interests were not properly protected, such contracts can be voided based on this. The Jeweler is also expected to have significant knowledge on the values of jewelries and could properly estimate the value of the stone as at the point the transaction was made. Again the contract can be voided on the grounds of fraud in the inducement. Since he chose not to properly represent the value of the stone to an incompetent party.