Jewelry made with the gemstone agate. Agate is one of the most varied and desired forms of chalcedony. Chalcedony is one of the many varieties of quartz. Scientifically classified as a cryptocystalline or microcrystalline quartz, it has a hardness of 7. Quartz, in all its forms, is the single most abundant mineral on earth, making up almost 12% of the earth's crust.