AGATE or CHALCEDONY
CRYSTALLOGRAPHY: Hexagonal, microcrystaline.
REFRACTIVE INDEX: 1.544 - 1.553
SPECIFIC GRAVITY: 2.651
HEAT SENSITIVE: No
Agate or Chalcedony (kal SED' uh nee) is any form of microcrystalline or cryptocrystalline quartz
(meaning any form of Quartz whose crystals are too small to be seen without high magnification).
In common practice, only the translucent, single color types are sold as 'Chalcedony',
whereas the rest of this group are sold as Jasper, Agate or under Variety names.
While definitions overlap, Jasper usually refers to an opaque, solid colored stone, while
Agate is defined either by its translucency, or by having a pattern to its colors.
Agate is often distinguished by multiple colors and can be quite vivid.
Agates are sometimes opaque, but they can be translucent or completely transparent.
Banded agates are found all around the world, with Brazil being one of the most productive sources.
They come in a large range of colors, detail and character of banding.
Lace agate is noted for its delicate designs and wide range of colors.
A more figured and colorful form is called Crazy Lace. Mexico is the main source for this material.
DENDRITIC, MOSS and PLUME AGATES:
These agates contain mineral inclusions which may be any color, but share a roughly
tree-like or branching form (dendron = tree). Those which have a plant or feather-like
appearance are called Moss or Plume agates, respectively.
Some of the most treasured gems are those that show a picture that mimics nature.
Oregon's Biggs Jasper is now the most common source. Bruneau Jasper, from Bruneau Canyon, Idaho,
used to be the preferred material. Gems from this locality frequently had blue "skies" which the Oregon
material lacks. Unfortunately, a dam has submerged the mining site and the material is now quite rare.
Fire agate is a brown, microcrystalline quartz which has a botryoidal (grape-like) growth form.
It contains layers of plate-like crystals of iron oxide (limonite) in various planes within it.
The iridescent colors of red, gold, green and rarely, blue-violet, result from interference between
light rays traveling through these thin layers. Usually, fire agate pockets occur within specimens of
colorless, white, or light gray chalcedony. It is found only in the American Southwest and Mexico.
Jasper is an opaque, solid or patterned variety of cryptocrystalline quartz which consists of tiny quartz
crystals colored by various mineral impurities. The various names of jaspers come from their color,
pattern or place name. All types take an excellent polish and are hard enough for all jewelry uses.
It is usually cabbed or carved. Most Bloodstone comes from India. All Mookaite is from Australia
Crocidolite or blue asbestos alters to a quartz material while retaining its fibrous structure.
This material is frequently stained by iron, giving it a golden brown color, known as Tigereye.
The original unstained stone is often known as Hawks Eye. The gem can contain both colors.
In this description, chalcedony will mean any translucent, cryptocrystalline quartz with a single
color, whether it has a special variety name or not. The types vary in color due to metallic
impurities, such as iron, nickel, copper and titanium that are present during crystallization.
This material is usually cabbed or carved. Sometimes, near transparent pieces may be faceted.
Carnelian material ranges in color from yellow-orange to rich and reddish orange
and/or orangey browns. It can vary from semi-opaque to highly translucent.
Carnelian is the only type of chalcedony which is regularly enhanced.
Iron is the source of its color and it can be easily heat treated to darken
red tones as the iron is oxidized. Most commercial carnelian comes from India.
Apple green chalcedony that derives its color from nickel is chrysoprase. Ranging from
nearly opaque to nearly transparent, its color spectrum includes olive to nearly pure
greens of medium tone. Very fine, highly saturated pieces have been successfully
misrepresented as Imperial jade. Most chrysoprase sold today comes from Australia.
Prase is a darker and more rare form, which comes from Eastern Europe. Small amounts
of green chalcedony colored by chromium (called Mtorolite) is found in Africa.
Marketed as "Gem Silica" this relatively rare, blue to blue-green, opaque to near
transparent material is the most expensive type of chalcedony. Found almost
exclusively in Arizona, it's color is due to copper. This material has an extremely rich color
(often found only in the softer Chrysocolla) and has the durability and hardness of quartz.
Available in various blues (generally designated by place names). They vary in depth of
blue color and the degree to which the blue is modified by gray or pink hues.
They vary in tone and degree of translucency. Some pieces have a slight adularescence
that enhances their value. This phenomenon, strongest in Moonstone, is due to light
interference from layers of microscopic inclusions and appears like a shimmering interior light.
Blues are from California and Nevada, respectively and are light-to-medium grayish blue.
Blue chalcedony from Namibia or 'African Blue' varies from gray-blue to almost pure blue.
The most unusual type is from Oregon. Its blues are modified with varying amounts of pink,
making a noticeably lavender gem known as "Holly Blue."
Turritella agate is composed mostly of turritella shells, fossils embedded in agate.
Iris agate shows iridescent colors reflecting from between the color layers.
Sard is similar to carnelian, but with a brownish tone and is more opaque.
Prase is a green or yellow-green chalcedony.
Plasma is a dark green, opaque variety. It usually has white or yellowish spots.
Bloodstone, or heliotrope, is plasma with red and orange spots of iron oxide.
Onyx is a chalcedony with straight bands of colors. Black Onyx occurs in thin bands. It is often dyed.
Sardonyx is onyx with white and red layers.
Flint and chert are opaque, dull gray or white. They rarely make an appearance as gems.
Petrified wood and dinosaur bone are primarily chalcedony in their modern composition.