| The oval measures 200mm high by 150 mm across
Gem portraits are usually tintypes (ferrotypes) and often have rust staining showing on the back label.
Presented in simple mounts intentionally carte d'visite size to fit in an album. Primarily it was the process which made them cheap not the size.
I would suggest that these were often taken with the intention of being given away usually to a friend or admirer.
A Gem portrait would qualify as a trifle and as such an acceptable keepsake.
Even Edwardians would not read too much into the giving and accepting of such a trifle as a Gem portrait.
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