Sorry for the loss of your Grandmother. This actually sounds like a probate issue. You did not say if it was a specific bequest in the will to you. If it is, you should probably hire a probate attorney and seek to resolve it that way. It really just depends on where your ownership rights came from and if you do have a right to the ring.
This response does not create an attorney client relationship nor should the advice be relied upon because it is not specific to your legal situation. Before you depend on legal advice, you should retain competent counsel.
Theft and conversion are torts. Whether you actually have a case for either of these causes of action is not clear from the information you provided. As the previous answer stated, it may be that you have a probate issue more than a civil litigation issue. I suggest that you speak with a local attorney and show him the basis for your claim as quickly as possible.
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Conversion is a tort. The problem I have after reading your narrative is whether you had the right to control and possess the ring. Without that, there is no action for conversion. If it was in your grandmother's box, why did you have more right to it than your mother?