Rockwerks Lapidary Forum

Hre is some gleened info on quartz, agate, jasper, etc, it's from a couple sources. This subject is one of the more confusing when you are trying to learn all this stuff.


Is That Piece of Jasper an Agate?
by NatureLady @ 3:42 am.

Is That Piece of Jasper an Agate?

Finding a great agate or piece of jasper is a thrill for any rockhound, but being able to identify exactly what it is that has been found is quite a headache for the beginner. These headaches can be relieved very easily though with just a little bit of knowledge about the different quartz group stones.

Agate and jasper are actually chalcedony, which in turn is cryptocrystalline quartz. All are SIO2. When you pick up a stone you can rule out that it is a piece of regular massive quartz quite quickly just by looking to see if you can see the grains of the stone. If you can see grains, you do not have an agate or jasper. Most likely, what you have then is massive quartz or some other type of stone. Many new rockhounds will mistake massive quartz for a piece of agate, so don’t feel bad if you do. It’s a very frequent mistake.

Jasper and agate will appear to be made of wax. If the rock is just plain clear to white translucent with no markings or patterns, it is considered chalcedony. If it is opaque, that is, if you cannot see into or through it, it is jasper. Jasper is most frequently earth tones or red but you can find jasper in just about any color or color combination and it can contain some very lively patterns. One well known form of jasper is called “picture” jasper, and just as the name suggests, the lines and markings look just like a scenic picture of mountains and valleys or forests and so on. Geometric patterns are also common in jasper stones.

If a stone is an agate, it will be translucent as is chalcedony, but an agate will have patterns. Most commonly, agates have bands, and are appropriately called banded agate. Sometimes the bands are also translucent, sometimes some are opaque. There are many agates named to describe how they look, such as plume, orbicular, or flower and many that are named for the place they are found, such as Dryhead or Lake Superior. For instance, moss agate is a clear to semi-clear agate that looks like moss was embedded in the stone. No two agates are alike and many fantastically patterned stones will not have specific type or place names.

There are also stones which you will find that have both jasper and agate in them. Both the opaque and translucent parts of these stones will appear waxy. These are often referred to as jaspagate. Once you become familiar with the look of both jasper and agate, you will be able to recognize jaspagate with no problems. One other stone that can be confused with agate or jasper is opal.

Opal will have flashes of color if it is precious opal. It can be also be common opal which is plain translucent or opaque and a just about any color or a mix of colors. Opal generally looks more glassy than waxy, and it is much more brittle and breakable than agate or jasper.

If you still aren’t sure when you find a rock if it is jasper, agate, or opal, you should take it with you and ask someone about it. Your local rockshop or club or even a jeweler’s shop can identify it for you. You will have few problems identifying these stones after the first or second time. Once you learn to identify these basic stones, you will be surprised how many different types of gemstones you will start noticing on your hunts.




by: mineralminers.com

PHYSICAL PROPERTIES

* Agate Chemical composition: SiO2
* Class: tectosilicate
* Crystal system: Hexagonal-R; 32 (trigonal-trapezohedral)
* Crystal habit: usually as banded or layered botryoidal masses with microfibrous structure
* Zoning: color banding and layers of inclusions are common
* Specific gravity: 2.57-2.64
* Index of refraction: 1.53-1.55
* Birefringence: weak, 0.005 to 0.009
* Hardness: 6.5 to 7
* Color: frequently colored by impurities and then may be any color, most agate used for ornamental purposes is colored by artificial means.
* Luster: dull vitreous to greasy
* Transparency: translucent, can be transparent or opaque
* Cleavage: Cryptocrystalline quartz shows no cleavage, although parting may occur along layers and splintery fracture can develope parallel to microfibrous orientation.
* Fracture: conchoidal to splintery
* Streak: white

Return to the Index of Agate Information Topics
QUARTZ (Agate) BACKGROUND INFORMATION

Agate is a cryptocrystalline variety of the mineral Quartz (SiO2). Quartz is the most abundant single mineral on earth. It makes up about 12% of the earth's crust, occurring in a wide variety of igneous, metamorphic and sedimentary rocks.

Quartz varieties are commonly separated into two groups based on the size of the individual grains or crystals; macrocrystalline quartz in which individual crystals are distinguishable with the naked eye, and cryptocrystalline quartz in which the individual crystals are too small to be easily distinguishable under the light microscope.

Some of the macrocrystalline quartz varieties are: Amethyst, Ametrine, Cat's-eye Quartz, Citrine, Phantom Quartz , Rock Crystal, Rose Quartz, Rutilated Quartz and Smoky Quartz.
Blue Aventurine Quartz and Green Aventurine Quartz are actually quartzites (a rock, not a mineral) composed essentially of interlocking macrocrystalline quartz grains with disseminated grains of other color imparting minerals.

The cryptocrystalline varieties of quartz may be separated into two types; fibrous and microgranular. Chalcedony is the general term applied to the fibrous cryptocrystalline varieties. Agate is an example of a fibrous cryptocystalline banded chalcedony variety of quartz. Carnelian, Chrysoprase and bloodstone are other chalcedony varieties.
Chert is the general term applied to the granular cryptocrystalline varieties of quartz, of which flint and Jasper are examples.

Return to the Index of Agate Information Topics

OCCURRENCE AND DIAGNOSTIC FEATURES

Agate is a variety of chalcedony, which is the general term applied to fibrous cryptocrystalline quartz varieties. It is a mineral generally of secondary origin, forming in cavities and veins by deposition from meteoric groundwater containing gelatinous silica from the weathering of silicate minerals. It can also form by deposition from late hydrothermal solutions at or near the earth's surface under conditions of low temperature and low pressure. Agate is commonly associated with siliceous volcanics, occurring as vein or vesicle filling. It occurs as a petrifying agent in fossil wood and bone. Agate is frequently found lining or filling rock cavities and fissures, sometimes forming geodes.

Agate is characterized by its occurance, hardness, fracture, specific gravity, refractive index and luster.

Return to the Index of Agate Information Topics

HISTORICAL INFORMATION AND USES

The name agate is from an old locality on the achate river in southwestern Sicily.

Agate has been used as gemstones and other ornamental objects for thousands of years.

In medieval times, the wearing of agate was said to bring God's favor and make one agreeable and persuasive, giving victory and strength to the wearer and protection from all dangers. Wearing agate was also thought to give pleasant dreams and cure insomnia.
Early Greeks made amulets of agate for protection from the elements of the sea.

Some of the earliest primitive stone tools (axes) fashioned by man's ancestors 2.5 million years ago in the Omo valley in Ethiopia, were made of quartz varieties such as agate. These materials were used because of their hardness and their isotropic brittleness which made it possible to shape the tools with relative ease.

The astrological sign of agate is gemini.
Agate is a birthstone for the month of May.
Agate is the symbolic gemstone for the 12th wedding anniversary.

Return to the Index of Agate Information Topics
METAPHYSICAL PROPERTIES

Agate is said to enhance one's perceptiveness and to stimulate analytical capacity, and to provide balance between one's physical, emotional, intelectual and spiritual states. Agate is also said to alleviate hostilities and promote goodwill.

Agate is said to help treat disorders of the pancreas and to improve circulation.

For more in-depth metaphysical information, see our Metaphysical Books section.

Just a few remarks that might be helpful.

Moss agate is what I consider a good example of jaspagate. It is basically opaque, but the presence of clear or translucent chalcedony is obvious. Some Agate Creek and Argentine agates are also basically opaque as well, but common usage tends to refer to chalcedneous stones exhibiting layer lines as "agate" even though they may be opaque.

Quartz comes in several common forms: crystaline, which is the simplest and weakest crystaline structure (optical quartz and amethyst are examples); quartzite, which has a slightly more complex structure and definitely grainy appearance, and is probably a first stage in the metamorphesis to chalcedony; and massive quartz, which is often mistaken for agate of the same hardness, and has less noticeable grain structure than quartzite but is quite prone to fracture because it is not quite microcrystaline. Chalcedony, and its agate and jasper forms, is about the same hardness as quartz (around moh7), but far less prone to fracture because of the strength and toughness of the microcrystaline structure.

Opal is slightly different chemically in having "water of hydration", and is not a chalcedony. It actually has hydrogen and oxygen chemically bound to the SiO[sub]2[/sub] in the same ratio as in water. This water of hydration can be driven off at high temperatures, leaving a lightweight quartz. It is a softer material as well, similar to obsidian in hardness. You can scratch an opal with a quartz or chalcedneous stone, but not vice versa.

Obsidian, or volcanic glass, is also SiO[sub]2[/sub] that has no organized crystaline structure. It is basically a supercooled liquid. Like all glass, it flows over time, even in its apparently solid state. This phenomenon is evident when you look though the glass windows of a really old house. The waviness was not there when the glass was new.

Thanks, this was really informative. This really helps the rock clueless like me.

Here's a diagram that I use for my students.
Bob

Hey Bob,
That's good! I will have to remember the silicate tree.
Thanks
Dan

Very good, Bob, but most sources I visit tend to group jasper and agate as species of chalcedony. I realize, however, that the term "chalcedony" can have both specific and generic meaning.