16.6.1.1: Agate

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May 6, 2022
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- 16.6.1: Chalcedony
- 16.6.1.2: Arkansas stone

gemology
Gemology Project

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Agate
Chemical composition	Silicon dioxide SiO₂
Crystal system	Trigonal
Habit	Aggregate
Fracture	Conchoidal
Hardness	6.5-7
Refractive index	1.535 - 1.539
Specific gravity	2.60
Lustre	Vitreous

Agate is a variety of banded microcrystalline quartz, or chalcedony. There are countless varieties of agate usually named after their physical characteristics or location of origin. The layers of agate form as layers of chalcedony line a cavity (or vug) in a host rock. Since the 12th century, agate has been attributed with the power of improving one's disposition as well as their eyesight. These beliefs still carry on today. One claim for agate was stated in The Book of Saxon Leechdoms written in 1864, where it was written that agate prevents harm from thunder, sorcery, demonic possession, poison, and drunkenness!

Varieties

Moss Agate

This variety is called "moss" agate because of the inclusions (chlorite, clays, various oxides/hydroxides, etc.) in transparent/translucent chalcedony. The final effect makes the stone resemble "moss". Depending on the inclusions the moss can show different colors, with green/brown and red as the most common. They usually take a good polish and make attractive stones.

Priday Moss Agate, Oregon

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Figure $\PageIndex{1}$ : Moss Agate from a Priday thunderegg (Oregon). Green moss.

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Figure $\PageIndex{2}$ : Detail of a Moss Agate from a Priday thunderegg (Oregon).

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Figure $\PageIndex{3}$ : Detail of a Moss Agate from a Priday thunderegg (Oregon).

Moss Agate (from 'thunderegg')

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Figure $\PageIndex{4}$ : Moss Agate from Oregon, red moss.

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Figure $\PageIndex{5}$ : Moss Agate from Oregon, red moss, detail.

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Figure $\PageIndex{6}$ : Moss Agate from Oregon, red moss, detail.

Cathedral Agate

The "Cathedral agate" is found in Mexico and features red and gold moss/plumes in a light purplish or white agate with very fine fortification (typical banding pattern).

Cathedral Agate, Mexico

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Figure $\PageIndex{7}$ : Mexican Cathedral Agate cab

Figure $\PageIndex{8}$ : Cathedral agate, detail of the red moss/plumes and purplish agate fortification

Figure $\PageIndex{9}$ : Cathedral agate, slab.

Crazy Lace Agate

This agate comes from Chihuahua, Mexico. This agate shows a very fine banding, often with striking and contrasting colors. Another interesting fact about this kind of agate is that the banding often grows parallel to the surfaces of the crystals that were present before the crystallization/deposition of the agate (usually calcite, quartz, zeolites). The presence of these layers provides very interesting patterns in the slabs and in the finished cabochons.

Crazy Lace Agate, Mexico

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Figure $\PageIndex{10}$ : Crazy Lace Agate cabochon

Figure $\PageIndex{11}$ : Photomicrograph showing the fine banding ("fortification")

Figure $\PageIndex{12}$ : A slab of Crazy Lace Agate showing the typical structure of this material