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| One
of the most widely available and frequently collected materials in
the world is chalcedony. Chalcedony, in all it's many forms, is a
fibrous, cryptocrystalline form of quartz. Most often intergrown with
the mineral moganite, another silicone oxide. Thus it is technically
a rock and not a recognized species of mineral. The name chalcedony
comes from the ancient Greek town of Chalkedon in modern day Istanbul.
Chalcedony is a favorite with lapidary enthusiasts as most varieties will take a superb polish to create stunning material suitable for use in jewelry and other artistic and collectible displays. It is usually semi-transparent, translucent or opaque. Chalcedony occurs in an astounding number of varieties that differ in appearance and color due to unique accessory minerals and the divergent environments in which they are formed. Some of the most popular varieties are outlined below. |
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Agate Agate comes in a variety of colors but can always be distinguished by its concentric banding. Many other forms of chalcedony are often referred to as agate (ie. moss agate) but only chalcedony that exhibits concentric banding patterns can be called a true agate. |
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| Moss Agate This form of chalcedony is marked by inclusions of other minerals that sometimes resemble moss. Usually oxides of iron or manganese, these inclusions sometimes form as dendrites and produce intricate plant or fern-like patterns. |
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| Jasper Jasper is a dense, opaque variety of chalcedony which gets its coloration from iron oxides. It is usually a red or brown color. Sometimes yellow. |
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| Onyx Onyx refers to a starkly colored black and white banded variety of chalcedony. Widespread misuse of the name onyx has resulted in it being used to describe similar looking materials such as banded calcites and marbles. Only this variety of chalcedony is true onyx. A closely related brown and white variety is known as Sardonyx. |
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| Chrysoprase Usually apple-green but sometimes dark-green, chrysoprase gets its hue from trace amounts of nickel oxides or silicates. There is a slightly duller colored variety of green chalcedony that owes its color to the presence of hornblende and/or chlorite which is known as prase. |
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| Carnelian The presence of hematite gives carnelilan its reddish brown appearance. During Roman times, this stone was fashioned into rings that were used to seal documents as wax does not stick to carnelian. |
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| Heliotrope Also referred to as bloodstone, this dark-green to emerald-green variety of chalcedony is decorated with inclusions of red jasper. A similar variety that has yellow jasper inclusions is known as plasma. |
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| Flint Flint is hard, most often dark grey, black or dark brown in appearance and has a waxy or glassy luster. It occurs chiefly in large masses and nodules in sedimentary environments. Flint can be "knapped" into a variety of shapes and can be made to have a very sharp edge. It has been used extensively in the making of stone-age tools. |
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| Chert Chert is similar to flint but is usually much lighter in color and is formed in a slightly different environment. Chert also has a duller luster and is more grainy in appearance. It sometimes contains small fossils. |
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| Pseudomorphs Chalcedony can often be found as a replacement to other minerals or materials. Most notably bone, petrified wood and some other fossils. Such materials are often refered to as being "agatized". While this is a fairly accepted term, it can be somewhat misleading. Without the signature concentric banding, it is most certainly not agate. |
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