I know they exist in Carnelian, Agate, Bohemian Glass and various metals ..
They are called " Talhakimt " and also " Tanfouk "
Worn in the Sahara by Tuareg women ... but i cant find out much more about them ?
The oldest example i have ever seen is a hand carved carnelian ring of the same shape kept in the museum of Aswan in south Egypt and labelled as Pharaonic....
Yours are likely part of the latest production and are not strictly from the more Saharan Tuareg but rather Sahelian and very common among the Wodaabe/Bororo peul people from Niger and othertuareg related groups.
As a thumb of rule, to be able to read most of the saharan/Sahelian/tuareg...crosses, rings and pendants you have to put them upside down in a way that the bail or the hanging hole should be at the bottom:
You will likely see a star, a sitting women, a frog's head, an abstract form, even a phallic symbol........the meaning of each is related to ancient beliefs and also to local people's cosmogony and how they embodies their respectiv "beginning of the world/life" myths.
Of course these subjects overlap the day to day most crucial issue for traditional people which is fertility.