Microcrystalline quartz is made up of such small crystals that they cannot be seen by the naked eye. They are also called cryptocrystalline. They come in two general forms: fibrous and grainy.
Fibrous – quartz crystals are arranged parallel to each other
Agate is translucent which means light can shine through it. This makes it different from jasper which is opaque. They can be any color caused by the impurities picked up by the quartz. Color banding is usual in agate and can be wall-lining banding (like the layers in an onion) or horizontal banding.
In the wall-lining banding sometimes impurities are pushed up by the growing crystals and end up being bands of color between growths of crystals. These and horizontal banding can be found in many of our local thundereggs.
Some of the more famous agates are Montana, Brazilian, Botswana and Lake Superior.
Translucent and a bright orange to red, this quartz is highly sought after. The color comes from iron impurities. Sometimes duller carnelians are heat treated to bring out a brighter color. The best carnelian is red throughout but most only has an orange or red exterior and the inside is clear.
Chrysoprase is one of my favorites. Its green color comes from the nickel impurity willemsite in the serpentine rock. Most chrysoprase comes from Australia but some can be found in Poland.
This rock originally formed as asbestos. Then the quartz and iron oxide comes along and transforms the asbestos into a beautiful peudomorph.
When wood is buried by a volcanic eruption the wood cells can be replaced by silica rich fluids and petrified. Also if the wood rots and leaves a cavity, that can be filled by agate or jasper.
Chalcedony is a dense, usually botryoidal in form, type of quartz. It is not transparent nor is it opaque. Two examples are chalcedony roses or fire agate. The term can have many meanings depending on the context though. Some consider chalcedony anything but jasper and chert. Others consider it anything not jasper and chert but also anything that does not fit in all the other classifications above.
Grainy – quartz crystals are arranged randomly
The differences in colors are caused by impurities and inclusions of organic material, metal compounds, etc. Sometimes cavities can be found in flint and chert in which small quartz crystals have grown.
Some nodules of flint can be found with a white coating which is silica. It has a concoidal fracture and has been used by the Native Americans, along with other microcrystalline quartz materials, for making arrowheads and spear points.
Flint is found in sedimentary rocks and is composed of the skeletons of marine organisms cemented together with silica to form concretions.
Jasper can be banded, spotted with other colors such as the green jasper with red spots known as bloodstone. Ocean jasper is particularly beautiful with its bands and orbicular formations.