Are there particular dates when the hallmark K18 was used?

by D.
(Oregon)

I recently purchased a cameo which was hand-carved on a shell. The cameo depicts several horses in front of a town. It has a brooch pin in the back and two loops so that it can be worn as a necklace. It has a hallmark of K18 on the gold bezel and a signature that begins with an "F" on the front. The name may possibly be something like "Fowl" for "Franklin."

I'd like to get an idea of the period of the piece and the artist.

Can you help me?