Mexican Agate is comprised largely of cryptocrystalline silica. This is the same mineral that makes up quartz, but chalcedony is most often a mixture of quartz and its polymorph moganite at the microscopic level. The various impurities which make their way in are the source of the stone’s coloration.
These agates vary widely in color. While some material consists only of monochromatic white, grey, and black spots, others may bear green or blue bits. The most prized have red and pink worked into them, creating dramatic coloration. For most of us, the red variety is the first one that comes to mind.