Agate

WHAT IS IT?

  • Transparent or Semi-transparent Chalcedony (microcrytalline quartz) often found in combination with crystalline quartz (Si02).

  • May be colorless or have colors (due to trace elements such as iron and manganese)

  • May have banding

  • Mohs Scale Hardness: 6.5 to 7.0 (between the feldspar and quartz)

  • Differs from Jasper - Jasper is always opaque, but Agate has some transparency

  • The name comes from the Greek name, Achates, for a river in Sicily where the Greek philospher Theophrastas discovered agates in the 3rd to 4th century BC.  Today, it is called the Dirillo River.

HOW DID IT FORM?

  • Formed when a rock cavity is filled with silica-rich water causing a deposition of silica onto the cavity walls.

  • May be formed in volcanic rock (igneous), metamorphic rock, or sedimentary rock

  • An organic object (wood, plant material) may decompose leaving a void in a sedimentary formation.
    If silica-rich water is present, an agate may form in the void.

  • A void in a lava flow (a vug) is another protential site of formation.  In this case the usual source of silica is volcanic ash.

  • Due to the durability of silica, agates often weather out of the surrounding rock.

  • Geodes are weathered out agates that have a solid interior

  • Thundereggs are similar to geodes, but the interior has a void (the silica didn't completely fill the vug)

WHERE IS IT FOUND?

  • Agates are found throughout the world

  • The states in the Pacific Northwest and vicinity have an abundance of agates (Washington, Oregon, Idaho, Montana, Wyoming)

  • These are some of the collecting sites in Washington State:  Damon Point (near Ocean Shores), Rialto Beach (near La Push), Whidbey Island, North Beach (near Port Townsend), Glass Beach (near North Beach), Beaches along the Columbia River, ancestral river drainages (Columbia, Salmon, Clearwater, Snake Rivers)

WHAT TYPES ARE THERE?

(Agates are classified by there features & where they are found.  There are too many types and sub-types to list all of them here)

  • Binghanite - only found in the Cuyuna iron range of Minnesota

  • Botswana agates - are a variety of banded chalcedony with predominantly shades of pink & gray from the country of Botswana in southern Africa.  Muted brown and apricot coloring is also representative.

  • Brazilian agate - geodes originating in Brazil that have brown-orange, white, and/or gray banding.  Often very large.  Can occur with crystalline quartz.

  • Carnelian agates - are translucent orange / red / brown chalcedony agates that having banding consisting commonly of red to orange chalcedony interleaved with white agate bands.

  • Coldwater agates - formed from the silica-rich gels found in some limestone & dolomite formations.  Not of volcanic origin.
    (example: Lake Michigan Cloud Agate)

  • Condor agates - are brightly banded agates that were discovered by Luis de los Santos in 1993 in the mountains near San Rafael in the Mendoza Province of Argentina.  At their peak of production about 1 ton of these agates was mined per month.

  • Coral agates - contains agate formed in the voids of ancient corals.

  • Dendritic agates - have fern-like features produced by the presence of manganese & iron oxides

  • Enhydro agates - contain water filled inclusion, sometimes with air bubbles.  Enhydro agates are still in the process of formation.

  • Eye agates - have one or more round concentric "eye" markings.  They are rare.  Eye agates are found in the Great Lake region of the U.S. & in Bostwana.  They are also found in Scotland, India, China, India, Egypt, Madagascar, Mexico, Uruguay, Brazil & Italy.

  • Fairburn agates -  are found in South Dakota & Nebraska.  They have parallel banding, holly shaped patterns, with a smooth exterior texture and thumb print indentations.  Some have cracked surfaces.  It is found in many colors, but is rare.

  • Fire agates - have layers of clear agate & hematite that produces an iridescent "fire"

  • Fortification agates - have concentric bands that resemble the defensive walls of a fortification.  The bands form closed concentric loops that radiate from a center point (there may be more than one center point)

  • Greek agates - Pale white to tan colored agates found in Sicily (a former Greek colony)

  • Iris agates - have a vibrantly colored rainbow feature produced by thin bands that refract the light

  • Lace agates - are banded chalcedony with complex lace-like patterns.
    (example:  Crazy Lace Agate from Chihuahua, Mexico)

  • Lake Superior agates - are iron stained agates found along the shores of Lake Superior

  • Moss agates - have green moss-like features.  Formed from chacedony and manganese or iron oxides (commonly oxidized iron hornblende)

  • Onyx agates - have parallel bands of agate and onyx (form of chalcedony).  Usually the band are black (onyx) and white, but the onyx can be of almost any color.  All forms of onyz have parallel bands.  Most agates have curved bands (this is an exception).

  • Patuxent River Stone - a red & yellow agate found in Maryland

  • Peeler agates - are banded agates that have a concentric shell structure.  Weathering causes them to have the appearance of a partially peeled orange, except that there are often more rind layers inside.

  • Petrified wood agates - contains agate that formed in the void in ancient log (ex. hollow logs)
    (example:  petrified logs in the Petrified Forest National Park in eastern Arizona)

  • Plume agates - contain plume-like features - fluffy inclusions which often appear to be soft & have depth.
    Sometimes the inclusiions resemble feathers, plants, or flowers.
    (example:  Graveyard Plume Agate)

  • Polyhedroid agates - have flat sided banding that forms polygons
    (examply:  Holley Blue Agate - a rare dark blue agate found near Holley, Oregon)

  • Sardonyx agates - contain bands of red sardonyx (a form of chalcedony)

  • Seam agates - contain chalcedony that has formed, by deposition, in the cracks and layers of a different type of rock

  • Tube agates - contain visible "tube" structures when cut lengthwise.  If the agate is cut along the top of the tubes, they appear circular.

  • Turritella agates - agate formed in fossilized freshwater Turritella gastropod shells

  • Waterline agates - have a series of parallel lines produced when the level of the silica-rich water changed during the formation process. 
    Commonly the change has tectonic origins.  Waterline agate are often found along with fortification agates.

WHAT IS IT USED FOR?

  • Jewelry & Ornamentals - pins, cabochons, brooches, paper knives, inkstands, marbles, seals, beads, carvings

  • Agates have been used in jewelry & decorations since the time of the ancient Greeks

  • Arts & Crafts

  • Industrial uses:

    • Knife-edge bearings for precision balances
    • Mortars & Pestles for crushing chemicals
    • Traditionally for leather burnishing tools

  • In ancient times, agates were a common material for hardstone carvings