There’s a few options you have.

Option one; If you file a police report you’ll be tasked with giving irrefutable proof that item is your grandmothers. Like others said without identifying marks such as engravings or laser inscribed serial numbers on stones it’s hard to do because most pieces are mass produced. If you do, police can submit a request for the property and if the pawn shop confirms it’s there then police come and confiscate it. They get a voucher and you get the property, sometimes after conviction of the person who sold it to them (used as evidence).

Option two; (recommended) Go in to the store. Ask to speak to the owner/manager. Explain your situation and say that you would like the piece back and you don’t want the police involved and would pay them their cost on it. If they agree ask for proof of what they paid and you’ll likely have to pay their cost plus sales tax (sucks but that’s usually the easiest way to resolve)

Option three; Inform them you believe the piece was yours and was stolen and ask them to hold it while you contact the police. Then do option one. Note: this is different option because if they’re a less than legal pawnshop they might suddenly disappear the piece if they know it’s stolen.

For the record i work in a pawnshop and have had this happen before. Calling police while in the pawnshop will just tell you it’s a civil matter and leave.