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About Minerals

Agates

A process which took nature hundreds, thousands, or perhaps millions of years to complete cannot be covered in a few words, but let's take a simplified look at their creation.

First came the raw materials-silica (silicon dioxide (Si02). This compound is a combination of silicon and oxygen and is a major rock builder in the earth's crust, being second only to the compound water in abundance. With silica so common, gems should be everywhere, but their formation depended upon conditions and this is where our story begins.

About 400 million years ago the Pacific Northwest began to rise from the sea. In a succession of volcanic upheavals a landmass rich in silica was created. Occasionally these lavas cooled so rapidly they formed a volcanic glass-obsidian. Generally however, volcanic masses cooled slowly, tended to crystallize and were porous, eventually breaking down to release minute particles of silica. This free silica in turn, was dissolved by acids in percolating ground water, then transported and finally deposited as concentrated liquids in subsurface cavities. Experts differ on how the liquids eventually became solids, however the following basic silica forms were the result.

Crystalline - Silica deposited in obvious crystal forms and known as quartz or rock crystal.

Opaline -Silica form similar to chalcedony, but totally noncrystalline, more porous and containing small added amounts of free water (3 t014 percent) held within its structure rather than its chemistry. Gem opals are of this form.

Cryptocrystalline -Silica deposits of microscopically fine-grained, almost non-crystalline masses known as chalcedony. Chalcedony characterizes agate and jasper.

Each of the silica forms occurred within cavities of previously formed rock and are termed secondary deposits. How each formed however, depended largely on where it formed.

Individual gemstones, when found in gravel bars or loose soil, are usually a long way from their birthplace, having rolled and tumbled with the eroding forces Generally the surface landmass of their origin has long since crumbled and vanished. Some Gems, however, are still embedded where they developed and show that each basic type resulted from its own characteristic surroundings.

Agate: The result of chalcedony which filled empty gas pockets or cracks in otherwise solid rock, forming as nodules or seams. Since only limited amounts of foreign materials were included, the stone is fairly pure and ranges from clear to translucent when held to the light. Not all agates formed this way, with the two exceptions being thunder eggs and fossils.

Recognizing and Testing Agate: The mineral world has many look-alikes, but not all have gemstone quality. To avoid filling your pockets with excess rock, three simple test can be made.

Luster: Look for even textured, non-granular material ranging from clear (glasslike) through translucent (semi-clear) to opaque (blocking out light).

Rough specimens may have a glassy, waxy, pearly, or even dull appearance and can be better judged when wet. Agate: Waxy, clear to near opaque. Jasper: Waxy to dull and opaque. Opal: Pear-like, clear to opaque.

Fracture: Broken edges are usually conchoidal (cupped, shell-like fractures as on a chunk of glass). Water-worn pebbles often show curved crescent-like surface markings. An exception to this is opal, which tends to be brittle and more angular in fracture.

Hardness: Part of the reason these are termed gemstones is in their ability to polish well and withstand abrasion. They rank between 7 to 7.5 on the Mohs hardness scale.

Probably no gemstone is known in a greater variety of colors and patterns than agates. And like fingerprints, no two of these are ever exactly the same. Recurring agate characteristics however, have been given appropriate names such as: White or Milk, Turtle-back, Coated, Snakeskin, Blue, Amethystine, Carnelian, Sard, Polka Dot, Sunset, Enhydro or Water Agate, Fortifaction, Iris or Rainbow Agate, Banded, Sardonyx, Grape, Eye or Orbicular Agate, Tube or Pipe, Angel Wing, Dendritic, Moss, Plume, Flower, Sagenite, and Ellensberg Blue.

via Gem Cutters News 5/05, via Quarry Quips 4/2000, via Breccia, 8/05