The pearl in this ring is from a yoag or horse mussel, a large seashell that lives on the sea bottom. In the early 20th century, Shetland fishermen dredged the yoags up for use as bait in the kind of line fishing in which they were then engaged. Each line had its own hook and several lines were fastened to a thicker piece of rope before being placed in the sea.
It was the women who baited the lines, but first they had to "redd" or untangle the lines with their fierce hooks then they had to open the large mussel shells and cut up the flesh for bait.
Occasionally, the women found pearls inside the mussels, but mostly those were small and quite dark in colour so got thrown away. This is an unusually large example which my mother found in the mid-1920s when she was about 16 and was baiting lines for a local fisherman. In fact she found two good-sized examples.
In the 1960s, she had those made into rings for me and my younger sister.
The gold used for the rings came from the strap of a gold watch our father had given her when they were young, but the watch was broken and no longer worth repairing.