The luscious peacock blue round drusy agate gemstone cabochon shown here is 29mm diameter. You can read more about what drusy agate is here.

Agate is traditionally believed to help you to discern truth, improve memory and concentration, increase stamina,  prevent insomnia and ensure pleasant dreams, enhance personal courage, protect you against danger, and provide a calming influence.

I used this cabochon to create this custom piece: