Chalcedony comes in many colors from the common tannish-grey often found on rock outcrops to greens and purples, reds and tans. People have given separate names to the various colors – bloodstone when there is spotted red (guess where that name comes from), carnelian when a bright red, a dirty brown chert when the quartz mineral is mixed with clay, bright apple-green chrysoprase, and jasper which is generally opaque. The naming is subjective – but these stones are among the oldest used in jewelry and some named as birthstones. You can find out more about the various types of silica in the Quartz: By Any Other Name.
Chalcedony, Jasper, and Agate are only three forms of very, very fine-grained quartz – the crystals are there, but so tiny that it exhibits none of the crystal shape – just the hardness of quartz with a translucent luster. Agate is banded or shows other structures and chalcedony is generally non-banded. Jasper is generally opaque due to inclusion of clay. The names are not important – they are made of the elements silica and oxygen, and each has the mineral structure of quartz.
Hesse, Reinhard. “Silica Diagenesis: Origin of Inorganic and Replacement Cherts.” Earth Science Reviews, 1989. https://doi.org/10.1016/0012-8252(89)90024-X
Pabian, R., Jackson, B, Randy, P and J Cromartie, 2006,Agates, Treasures of the Earth,Firefly Books, 184 pp.
Nevada, United States
This locality produced very clear, gemmy chalcedony as well as brownish and purple transparent to opaque material. The clear material is rare and very expensive. The purple color in chalcedony is attributed to iron impurities – just like the coarse grained equivalent of amethyst. Both are the same mineral species of quartz – they just crystallized differently.
Here is some text about this location.