Edmonton Tumblewood Lapidary Club
Edmonton Tumblewood Lapidary Club
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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.