AGATES
Prepared by Daniel W. Smith
Mineral SiO2
Specific Gravity 2.6 to 2.65
Hardness 6.5 to 7.0
Cleavage none
Fracture conchoidal, uneven
Colour Various and variegated
Transparency Translucent, opaque
Fluorescance varies
Agate (silica oxide, SiO2) is a part of the silica group of minerals in the category chalcedony. It is characterized by bands or zones of different colours. They appear to be named after the Sicilian River Achates, where agates must have been found in ancient times.
Agates are found in siliceous volcanic rocks as nodules or geodes (have a center cavity). The nodules and geodes vary in size from a few mm to several m in diameter. The bands form by cyclic crystallization. This may occur at the time of formation or possibly by gradual crystallization from a siliceous solution. The outside may become white due to weathering. When a hollow cavity develops, crystals can form in the void. These may be roch crystal, amethyst, smoky quartz, calcite, hematite, chalybite and/or zeolite.
Habit agate forms concentric or irregular bands often leaving a cavity.
Onyx is a form of agate with parallel, usually straight bands. Commonly used for making cameo broaches.
The chalcedony (SiO2) are a variety of silica dioxide which is comprised of minute quartz crystals with sub microscopic pores. There are two main varieties: chalcedony, a uniformly coloured material, and agate which has curved bands of different colours.
Various names based, to some extent, on colour:
Carnelian red to reddish brown to brown
Chrysoprase apple green
Heliotrope blood stone – green with red spots
Jasper red also yellow, brown, green and gray-blue
Moss agate milky white, bluish white to colourless with green, brown and black moss-like dendrite impurities of manganese oxide.
Flint and chert opaque forms, dull gray to black
Opal (SiO2 nH2O) no crystal system – amorphous
Hardness 5.5 to 6.5
Colour – variable
Agate varieties are also identified by their physical appearance as follows:
Eye agate Ring design
Layer agate Layer parallel to exterior.
Dendritic agate Translucent chalcedony with dendrites
Fortification agate Patterned like the ground plan of a old fortress
Orbicular agate Circular layers
Moss agate Translucent chalcedony with moss-like inclusions of hornblende
Scenic agate Scenery-like image – brown or reddish colouring and dendrite
Pseudo agate similar interior to geode but exterior is geometric shape
Tubular agate channels of flow filled
Sard stone Agate with parallel layers
Brecciate agate Broken agate pieces held together by quartz.
Many, but not all, agates can be dyed to red, yellow, black, brown, green or blue.
USES
Art objects, rings, brooches, pendants and cameos.
References:
Hamilton, W.R., Wooley, A.R. and Bishop, A.C. 1974. The Henry Holt Guide to Minerals, Rocks and Fossils. Hamlyn Publishing Company, New York, 320 p.
Schumann, W. 1979. Gemstones of the World. Sterling Publishing Co., Inc., New York, 256 p.
Prepared by Daniel W. Smith
Mineral SiO2
Specific Gravity 2.6 to 2.65
Hardness 6.5 to 7.0
Cleavage none
Fracture conchoidal, uneven
Colour Various and variegated
Transparency Translucent, opaque
Fluorescance varies
Agate (silica oxide, SiO2) is a part of the silica group of minerals in the category chalcedony. It is characterized by bands or zones of different colours. They appear to be named after the Sicilian River Achates, where agates must have been found in ancient times.
Agates are found in siliceous volcanic rocks as nodules or geodes (have a center cavity). The nodules and geodes vary in size from a few mm to several m in diameter. The bands form by cyclic crystallization. This may occur at the time of formation or possibly by gradual crystallization from a siliceous solution. The outside may become white due to weathering. When a hollow cavity develops, crystals can form in the void. These may be roch crystal, amethyst, smoky quartz, calcite, hematite, chalybite and/or zeolite.
Habit agate forms concentric or irregular bands often leaving a cavity.
Onyx is a form of agate with parallel, usually straight bands. Commonly used for making cameo broaches.
The chalcedony (SiO2) are a variety of silica dioxide which is comprised of minute quartz crystals with sub microscopic pores. There are two main varieties: chalcedony, a uniformly coloured material, and agate which has curved bands of different colours.
Various names based, to some extent, on colour:
Carnelian red to reddish brown to brown
Chrysoprase apple green
Heliotrope blood stone – green with red spots
Jasper red also yellow, brown, green and gray-blue
Moss agate milky white, bluish white to colourless with green, brown and black moss-like dendrite impurities of manganese oxide.
Flint and chert opaque forms, dull gray to black
Opal (SiO2 nH2O) no crystal system – amorphous
Hardness 5.5 to 6.5
Colour – variable
Agate varieties are also identified by their physical appearance as follows:
Eye agate Ring design
Layer agate Layer parallel to exterior.
Dendritic agate Translucent chalcedony with dendrites
Fortification agate Patterned like the ground plan of a old fortress
Orbicular agate Circular layers
Moss agate Translucent chalcedony with moss-like inclusions of hornblende
Scenic agate Scenery-like image – brown or reddish colouring and dendrite
Pseudo agate similar interior to geode but exterior is geometric shape
Tubular agate channels of flow filled
Sard stone Agate with parallel layers
Brecciate agate Broken agate pieces held together by quartz.
Many, but not all, agates can be dyed to red, yellow, black, brown, green or blue.
USES
Art objects, rings, brooches, pendants and cameos.
References:
Hamilton, W.R., Wooley, A.R. and Bishop, A.C. 1974. The Henry Holt Guide to Minerals, Rocks and Fossils. Hamlyn Publishing Company, New York, 320 p.
Schumann, W. 1979. Gemstones of the World. Sterling Publishing Co., Inc., New York, 256 p.