WHAT IS CHALCEDONY?
Chalcedony is a microcrystalline (meaning: consisting of or having crystals that are small enough to be visible only under the microscope) form of Quartz, and scientifically is not its own mineral species but defined separately from Quartz chiefly by appearance. Under Chalcedony, we have many different varieties who themselves are simply visually different forms of Chalcedony, different colours, patterns or in some cases occuring with different inclusions or impurities.
In the gemstone trade, the term Chalcedony is often used specifically to describe the white, gray, or blue translucent types of Chalcedony, but its technical term includes all additional varieties. In this article we’ll take a look at the different crystal varieties and names of Chalcedony, and how they are defined and identified. You might be surprised to see what well known crystals are actually Chalcedony (and thus, scientifically, Quartz) by a different name!
Multicolored banded variety, and the most well-known and unique form of Chalcedony. Agate itself has many different known varieties. Names of different Agate varieties include Dendritic, Tree, Moss, Blue Lace, Crazy Lace, Botswana, Fire, Eye, Coral Flower, Laguna and Turritella.
All Agates contain distinctive patterns, which make each Agate different from the next. A generally accepted requirement in the definition of Agate is that it has to be banded. This qualification distinguishes it from other forms of Chalcedony which are not banded.
Exceptions include Dendritic Agate and Moss Agate, which are not true Agates since they lack the banding patterns, but they are still traditionally called Agates since they have more than one color. Sardonyx and Onyx as banded material are both technically Agates.
Bloodstone / Heliotrope
The mineral aggregate Heliotrope, also known as Bloodstone, is given to various combinations of green material with red spots. If the material is translucent, it is called green Chalcedony, or Plasma. If it is solid opaque, it is called green Jasper. The red spots are classically formed by the inclusion of iron oxide, however can also be the result of Actinolite or Hornblende needle inclusions, depending on the material. Sometimes Plasma with white or yellow spots is also referred to as Bloodstone, mostly in the retail sector.
Chalcedony that is orange, amber-red to red in colour, and translucent. Some forms of Carnelian may also exhibit banding and can therefore be classified as both Carnelian and Agate.
Chalcedony that is opaque (ie solid colour, not translucent), pigmented by admixed iron oxides. Jasper, like Agate, has many different known varieties. It can be a single colour, multicolored, spotted, striped or speckled. Names of different Jasper varieties include Ocean, Dalmatian, Zebra, Brecciated, Dragon Blood, Fancy, Kambaba, Picture, Bumblebee, Rainforest and Polychrome.
May describe solid black Chalcedony, banded black and white Chalcedony (usually in parallel banding), or any Chalcedony with a black base and white upper layer. When banded, Onyx is technically a form of Agate.
Opaque, compact Chalcedony which contains small amounts of Mica, Hematite, or Goethite scales, causing a glistening or glittering effect when turned in the light. Green is arguably the most common, however orange, yellow and red Aventurine also occur.
Chrysoprase, Prase, Plasma
Chrysoprase, Chrysophrase or Chrysoprasus is a green gemstone variety of Chalcedony that contains small quantities of nickel. Its color is normally apple-green, but can go to deep green.
Prase is a light to dark emerald green, translucent to semi transclucent Chalcedony, with colouring caused from inclusions of green minerals such as Actinolite, Hedenbergite, Chlorite, or Malachite. Prase is usually a darker, less saturated color than Chrysoprase.
Plasma is the name given to dark green Chalcedony.
Sard is a name given to yellow to brown Chalcedony. The identifying differences separating Sard from Carnelian are that Carnelian is more translucent, and has tones that are more red than brown.
Sardonyx is Chalcedony that has alternating parallel bands of brownish to red with white or sometimes black bands. Since it is banded, it is technically a form of Agate.
Tiger’s Eye, Tiger Iron & Pietersite
Tiger’s Eye is a pseudomorph of compact Chalcedony Quartz after the fibrous mineral Crocidolite. It is formed when the Quartz takes over and dissolves the Crocodolite, leaving the Quartz in a finely fibrous and chatoyant form.
Tiger Iron is a combination of Tiger’s Eye (a Chalcedony pseudomorph), red Jasper (an opaque Chalcedony) and black Hematite.
The trade name of Pietersite is used for a fractured or brecciated Chalcedony containing amphibole fibers.