Silicon dioxide, better known as quartz, is one of the most abundant minerals on Earth. Mineralogists divide quartz into two classifications: crystalline and ryptocrystalline, based on the size of its crystals. Cryptocrystalline is also called microcrystalline quartz.

Due to its distinct crystal formation, crystalline quartz is usually transparent. Rock crystal, amethyst, citrine, and rose quartz are well-known examples of crystalline quartz.

The atoms in cryptocrystalline quartz pack together to form stones that are either opaque or translucent. The atoms take the form of fibers, rather than crystals, and the stones often contain water or air trapped between the layers of fibers.

The cryptocrystalline quartzes are informally divided into two groups: jasper and chalcedony (kal-ced-nee). The jaspers are often opaque, while the chalcedonies tend to be transparent. Most mineralogists don’t distinguish between the two, simply referring to both as cryptocrystalline.

Because there are too many cryptocrystalline quartzes to describe in one article, we’ll use the commonly accepted groupings. Onyx, prase, sard, sardonyx, and tiger’s eye are usually classified as jaspers. The chalcedony family generally includes agate, aventurine, bloodstone, carnelian, chrysocolla, and chrysoprase.

As described above, chalcedony is a group name. It also is the name of a particular stone within that group. Blue Mist chalcedony is a pale, almost white shade of blue that glows beautifully. It’s durable and hard, making it a good choice for jewelry.

Agate probably has the greatest variety of color and variety of any gemstone. It’s also one of the most abundant of all stones. Named after the Achates River, which flowed through Sicily in ancient times, agate takes a brilliant polish and has been widely used. Archeologists have found agates used by humankind more than 20,000 years ago.

Most mineralogists require agates to show banding (stripes), however many kinds of chalcedony without banding are commonly referred to as agates. Dentritic agate, which appears to have ferns or tiny trees enclosed within it, is one example. Moss agate and plume agate are also unbanded chalcedony.

Ancient peoples ascribed both magical and medicinal powers to agate. Placing an agate in the mouth relieved thirst, placing it against the forehead relieved a fever, and wearing an agate helped its possessor speak only the truth. Agate was also carved, especially into bowls. European museums have extensive collections of these bowls. Egyptians also carved agates into cameos more than 3000 years ago.

Aventurine is another chalcedony that carvers used extensively for bowls, vases, and ornaments. Although it is found in several colors, aventurine is usually green. It’s been mistaken for jade. Most aventurine sparkles, a phenomena known as aventurescence, due to tiny bits of other materials embedded in it. Mica flakes create a gold or silver glitter; goethite and hematite inclusions result in a red or green sheen; and fushite is responsible for a glistening green. As they did with most green stones, early peoples used aventurine to soothe the eyes. It was also valued for its ability to enhance the wearer’s mental acuity.

Like aventurine, bloodstone contains inclusions. Bloodstone, also known as heliotrope, is usually green with red spots. Deposits of iron oxide are responsible for the red, which does not glitter. One legend maintains that the red formed when drops of Christ’s blood landed on green jasper at the foot of the cross.

True to its name, bloodstone has a long history of healing bleeding and diseases of the blood. People used it to stop nosebleeds and bleeding from any kind of wound, by pressing it against the afflicted area. Martyr’s Stone was another name given to bloodstone, due to its extensive use by carvers to depict the crucifixion and similar scenes.

Carnelian,also called cornelian, is a well-known red chalcedony. Like bloodstone, its oranges and reds come from iron. However, the iron is distributed throughout the carnelian, rather than concentrated in spots.

Although used to stop bleeding, carnelian was more often worn to prevent skin diseases and insanity. It was also reputed to bestow courage and eloquence upon the wearer, and was especially recommended for those speaking in public.

Chrysocolla is usually a wonderful robin’s egg blue, very similar in appearance to the finest turquoise. Its hues can vary from bluish-green to green. Copper is the source of the glorious colors in chrysocolla. Pure chrysocolla is too soft to use in jewelry. Only when it is formed in conjunction with quartz can it be utilized for any purpose other than as a collector’s specimen.

Like most blue stones, chrysocolla is associated with peace and calm.

Chrysoprase, like chrysocolla, incorporates the Greek word for golden (chryso) in its name. Prase is from another Greek word meaning leek, and the two names describe chrysoprase’s pale yellowish-green color, a result of nickel impurities. Depending on the mineral content, chrysoprase may also be bright green. Some chrysoprase may be mistaken for jade.

Chrysoprase is the most valuable of the chalcedonies.

The chalcedonies are mined worldwide, including the United States. The stones are generally cut into rounded shapes (cabochons) and mounted in jewelry, or made into beads