QUARTZ

Rock Crystal transparent, colorless

Amethyst violet to red-purple

Rose quartz pink, rose-red or pale rose

Citrine clear yellow, yellow to red-orange or orange-brown

Smoky quartz (Cairngorm) smoky to grayish blacke or pale brown to black

Milky quartz milk-white

Aventurine grayish, yellowish, brownish, or green; glistening with

enclosed scales of mica or hematite

Cat's eye opalescent from inclusions of asbestos

tigereye with lustrous yellow to brown parallel fibers;

chatoyant

CHALCEDONY

Chalcedony: Cryptocrystalline quartz and much chert, commonly microscopically fibrous. The material of agate.

Berry and Mason's Mineralogy indicates that chalcedony is slightly porous and thus able to be penetrated by coloring agents.

From Hurlbut's Dana's Manual of Mineralogy:

"Chalcedony is the general term applied to fibrous varities (i.e., of quartz, ed.). More specifically it is a brown to gray, translucent variety, with a waxy luster, often mammilary and other imaginative shapes. Chalcedony has been deposited from aqueous solutions and is frequently found lining or filling cavities in rock. Color and banding give rise to the following varieties:

A. Fibrous varieties (i.e., of quartz, ed.)

Carnelian (Pale to deep red or red brown)

Sard (Deep orange or red brown)

Chrysoprase (apple green (colored by mickel))

Agate (alternating layers often of different colors)

Wood (petrified)

Onyx (layers in parallel planes)

Sardonyx (alternating black and white parallel bands of sard and carnelian, contradiction?))

Heliotrope or Bloodstone (Bright green with small red spots)

B. Granular varieties (i.e., of Quartz, ed.)

Flint and Chert (Flint found in chalk, Chert found in bedded deposits)

Jasper (granular with dull luster colored by Hematite)

Prase (dull green color)

So from this, Chalcedony is the general term and agate and the others are varietal terms.

AGATE

Mineral; variation of chalcedony.

Agates are a form of quartz formed by precipitation from watery solutions in rounded cavities in lava rocks. Agates are characterized generally by their banded, concentric paterns. They are unique from other forms of quartz (jasper, chert) because they are translucent (clear) rather than opaque.

Agate, n., 1. A varigated quartz in which colors are in bands, in clouds, or in distinct groups. 2. A kind of silica consisting mainly of chalcedony in varigated bands or other patterns commonly occupying vugs in volcanic or other rock. (From Dictionary of Geological Terms, from the American Geological Institute 1962 edition).

A personal note on AGATES: I have examined in detail hundreds of rough and thinly cut agates, especially the so-called Lake Superior variety common in N. Kansas and S. Nebrasksa. I have observed that almost all formed by precipitation on the wall of vugs, probably gas bubbles in volcanics. The semi-concentric circular patterns "grew" inward probably over many dozens or hundreds of years, with the final precipitation at the very center. It is often possible to identify the filled remains of the particular channel(s), often tubular, attached to each band that allowed the fluids to pass into the central part of the vug.

Additionally. almost any AGATE that has intense vivid blues, greens, reds, or unusual colors has been artificially dyed. It is relatively easy, for example, to dye colorless agate intense black by submerging the stone in a sugar solution for a relatively long period (weeks to months), washing the surface, and then dehydrating ("burning") the infiltrated sugar (within the agate) using concentrated sulfuric acid. I don't have the reference at hand, but there as a series of articles on such dye processes in Lapidary Journal 10 or 15 years ago, maybe 20 years ago by now.

CHERT

Sedimentary rock composed of variations of quartz.

1, Insoluble residue. Cryptocrystalline varieties of quartz regardless of color, composed mainly of petrographically microscopic chalcedony and/or quartz particles whose outlines range from easily resolvable to unresolvable with binocular microscopes ordinarily used. Particles typically 0.5 mmin diameter. 2. Mineral; a compact siliceous rock composed of chalcedony or opaline silica, one or both, and of organic or precipitated origin. ... Flint is a variety of chert ((From Dictionary of Geological Terms, from the American Geological Institute 1962 edition).

Return to Top - Return to Science

Redire- Return to the Roman Forum main arena.

© Copyright 1997 by Scott Grasse.