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: Agate

Agate used in Jewelry

Rough & Polished Agate

Source: Occuring worldwide

Agate is a fiberous, translucent to opaque, compact microcrystalline variety of chalcedony (quartz) that occurs in banded white, gray, brown, grayish-green and grayish-blue hues. Agates are formed by silica-rich water percolating through cavities and fissures in volcanic rock, and occur worldwide. Agate is a relatively porous material that is easily dyed to alter or enhance the color.

Most banded agates occur as nodules and geodes within ancient volcanic rock or lava, where cavities where filled by silicate and zeolitic minerals deposited in layers upon the walls of the cavity, with the first deposit forming the outer skin of the agate. When the deposition process does not proceed to the point of filling the cavity, a hollow space is left causing a geode (below, right). The interior of the geode can be encrusted with amethyst or quartz crystal. Banding found in Plume Agate, Sagenite, Thunderegg, and Tube Agate is caused by silicate (green) iron oxide or limonite (red), and is referred to as banded agate, riband agate or striped agate.

Banded Red Agate

Banded Red Agate

   Chalcedony Agate Geode

Banded Blue Agate

Dendritic agates (above, left) and moss agates (above, right) have delicate fern-like patterns that are formed from the presence of iron and manganese ions. Mexican agate or "cyclops agate" has an array of colors including black, brown, gold, green, and red embedded in chalcedony. So called "sparkling moss agate" is a reddish brown color with flecks of shimmering red and solid black.

Dendritic Agate

Dendritic Agate

   Crazy Lace Agate Cab

Zoom: Crazy Lace Agate

Both agate and chalcedony are totally opaque or translucent so they are typically cut into a cabochon or beads. Chalcedony was perhaps one of the first materials used by early man to form tools and arrowheads, due to its durability and abundance, and flint used as a fire-starting tool, is a variety of chalcedony.

Agate belongs to the tectosilicate quartz family of minerals that include chalcedony, carnelian or 'sard,' black onyx, and jasper. Agate is typically associated with igneous rock, grown in pegmatites and geodes that formed during the mountain-building process.

Agate (Microcrystalline Quartz) Chemistry & Physical Properties

Crystal System rhombohedral microcrystalline
Crystal Habit cryptocrystalline
Specific gravity (SG) 2.58 - 2.70
Mohs Hardness Scale 7
Toughness good
Fracture conchoidal
Cleavage none
Streak white
Chemical Composition SiO2 (Quartz)

Agate (Microcrystalline Quartz) Optical Properties

Optical Properties singly refractive
Refractive Index 1.530-1.540
Birefringence +0.004
Pleochroism none
Surface Luster dull to waxy
Diaphaneity transparent, translucent, subtranslucent
Gem Color blue, brown, gray, green, purple, white

The German village of Idar-Oberstein grew around the mining and cutting of agate and carnelian in the 1400s. Agate is often dyed or stained in a wild range of colors using color-fast aniline dyes. These dyes can fade when exposed to sunlight for long periods of time.

Crazy Lace Agate Geode

Crazy Lace Agate Geode

   Chalcedony Agate Geode

Chalcedony Agate Geode

Gemology Books
Gemstone Books

Bibliography and Reference on Agate

1. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.

2. A.C. Akhavan, Twinning in Quartz Crystals www.quartzpage.de

3. GIA, Characteristics of Citrine, Amethyst & Smoky Quartz www.gia.edu

4. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition

5. Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press

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