Agate | Gemopedia

Agate is the banded form of chalcedony, a cryptocrystalline variety of quartz. Rather than a single crystal, it is composed of a myriad of miniature crystals that can only be seen with extreme magnification. Agate's name is derived from the site of its discovery, the river Achates (now Dirillo) in southwest Sicily. You will often find striking curved or angular banded patterns of color flowing within agate, which are caused by the presence of various minerals, often iron and manganese. One of the oldest known gemstones, agate is a favored material of lapidaries and artisans around the world.
Colors
Almost All Colors

Agate Classification

Common Name

Agate

Species

Quartz

Variety

Chalcedony

Colors

Almost All Colors

Alternate Names

Gemstone Groups

Key Separations

Appearance, fracture, refractive index, specific gravity

Comments

Agate is a variety of chalcedony, a semitransparent to translucent polycrystalline quartz with curved, angular or gnarly bands or layers. "Agates form in nodules and veins in secondary container rocks" (Agates and Jaspers: Ron Gibbs). Typically carved or fashioned into beads, cabochons or sliced to show its banding. Dyeing is also sometimes employed for color or emphasis on the successive layers or bands. Fortification Agate: banded agate that is that have alternating parallel layers of varying colors. Plume Agate: has feathery appearance. Moss and Tube Agate: have a moss or tube like appearance. Sagenitic Agate: agates with secondary crystal replacements.

Agate Optical Properties

Transparency

Semitransparent - Translucent

Refractive Index

1.535-1.539

Birefringence

0.004-0.004

Optic Character

NA

Optic Sign

NA

Polariscope Reaction

Aggregate (AGG)

Fluorescence

SWUV: Inert
LWUV: Inert

CCF Reaction

Pleochroism

None

Dispersion

Strength: none

Comments

Agate Chemistry & Crystallography

Chemical Name

silicon dioxide (aka silica)

Chemical Formula

SiO2

Synthesis

Crystal System

Trigonal

Classification

Silicate

Nature

Natural

Crystallinity

Polycrystalline

Comments

Agate Characteristic Physical Properties

Hardness

6

Streak

Specific Gravity

2.55-2.7 Typical:2.6

Toughness

Excellent

Inclusions

Typical agate is described as having curved angular banding or layers, but it can also have a plume or moss like appearance. Bands may have a very fine grain or fibrous structure due to its microcrystalline structure and may vary in translucency. Agate is sometimes dyed.

Luster

Greasy

Stability

Good

Fracture

Conchoidal, Granular

Cleavage

None

Comments

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