AGATE is a mineral in the silica family, composed of the same basic stuff as quartz and other stones. And then there are many different types of agate. This particular agate formed in an area where marine critters (sea snails) known as turitella lived.
Turitella shells looked a lot like Bugles, but were MUCH lower in saturated fat. Anyway, I digress... 
As they died, their little bodies dropped onto the ocean bottom where the agate was forming. Of course, only their shells remained to be fossilized in the agate — no turitella guts, thank you — and their shells were turned every which-a-way on the pile. So, in a piece of this type of agate, one can see little circles or lines, or arcs, depending on how each shell was turned.
It might sound silly, but what attracted me to this one piece, over all the others that were available, was the "crescent moon" shape on the top. 
And here is the flatter, "bottom" of the stone:
Even as a child, I was fascinated by "pretty rocks" and have a small collection that I have found in nature or purchased in shops. If my followers are interested, I will share some of the others, the same way I shared this one. Hope you enjoy! 😊