This is not technically a cameo, but they are often referred to that way. It is a portrait miniature.
It is not possible to tell from the pix whether this was done with a transfer (decal) or is actually hand painted porcelain. Since your grandmother was a china painter (did she do it professionally or as a hobby?) it is natural she would have liked this type of jewelry. The numbers on the back would be model & perhaps inventory numbers, indicating a commercially produced piece. Mass produced pieces can still be hand painted. The porcelain manufacturers of Limoges made many pieces of this sort.
Transfers, when seen under magnification, typically have a stippled look, the little dots of color from the printing process. A painted piece will not look like this.
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Any information by: Darlene
I have a similar cameo. Did you find any information on this?