What is Opal?

Play of Color and Opalescence

Wonderful Names Used to Describe Opal

There are many types of opal and a wide variety of names are used to describe them. If you have spent a short amount of time looking at opal you have probably been surprised by the large number of wonderful names that are used to describe it. Names such as fire opal, black opal, jelly opal, boulder opal, matrix opal, Coober Pedy, Mintabie, Andamooka, precious opal, opal doublet, and opal triplet can ignite your imagination. Our goal on this webpage is to help you understand these names, where they come from and how they are used. We would also like to share some pictures of our favorite gemstone. Enjoy!

Precious Opal

"Precious opal" flashes iridescent colors when it is viewed from different angles, when the stone is moved or when the light source is moved. This phenomenon is known as a "play of color". Precious opal can flash a number of colors such as bright yellow, orange, green, blue, red or purple. Play of color is what makes opal a popular gem. The desirability of precious opal is based upon color intensity, diversity, uniformity, pattern and ability to be seen from any angle.

Precious opal is very rare and found in a limited number of locations worldwide. Most precious opal has been mined in Australia, secondary sources include: Mexico, Brazil, and the United States. Canada, Honduras, Indonesia, Zambia, Guatemala, Poland, Peru, New Zealand and Ethiopia. The black opal on the left was mined at Lighning Ridge, Australia and the white opal on the right was mined at Coober Pedy, Australia..

Common Opal

"Common opal" does not exhibit a "play of color". It is given the name "common" because it is found in many locations throughout the world. Most specimens of common opal are also "common" in appearance and do not attract commercial attention.

However, some specimens of common opal are attractive, colorful and lustrous. They can be cut into gemstones that accept a high polish. They can be beautiful but simply lack a play of color that would earn them the name "precious". Common opal is frequently cut as a gemstone and can command reasonable prices. Shown at right is a honey-colored opal from Mexico and teardrop-shaped stone cut from Peruvian blue opal.

Fire Opal

"Fire Opal" is a term used for colorful, transparent to translucent opal that does not exhibit a "play of colors". Instead, it has a bright, firey color that is present throughout the stone. Fire opal is usually orange to red in color, however, many people apply the name "fire opal" to stones that are a bright yellow color.

Fire opal is cut in a variety of ways. Transparent fire opals are usually faceted and translucent stones are usually cut into cabochons. The specimen on the left is a faceted fire opal cut from material mined in Oregon. It is 9 millimeters by 7 millimeters and weighs 1.2 carats. The yellow stone on the right is a faceted fire opal cut from material mined in Nevada. It is a 9 millimeter round that weights 1.7 carats.

Amphibolite

What is Amphibolite?

Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. It is composed primarily of amphibole and plagioclase, usually with very little quartz.

Amphibolite  is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are usually crystal cumulates. Rocks with >90% amphibole which have a feldspar groundmass may be a lamprophyre.

Amphibolite is a grouping of rocks composed mainly of amphibole (as hornblende) and plagioclase feldspars, with little or no quartz. It is typically dark-colored and heavy, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.

Amphibolites need not be derived from metamorphosed mafic rocks. Because metamorphism creates minerals based entirely upon the chemistry of the protolith, certain 'dirty marls' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolites (tremolite-schists, grunerite-schists, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalts create ortho-amphibolites and other chemically appropriate lithologies create para-amphibolites.

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Aside from Mid-Ocean Ridges, subduction zones, and collision zones, there are areas where the stretching or compression of plates causes slight fractures in the earth’s crust. These areas of crustal stress are called faults and there are essentially two types. Normal faults occur where tension within the Earth stretches the crust to form a basin, or range, with fault-block mountains flanking the basin. The southern Rockies include a basin and range area formed as a result of a normal fault. Reverse faults occur where compression squeezes the crust together as one block of land slides over another forming overthrust mountains. Impressive examples of overthrust formations can be seen in Montana’s ‘Glacier National Park.’

Another type of fault occurs where plates are sliding, shearing, or grinding past each other, folding mountains and producing earthquakes in the process. These plate boundaries are called Lateral, or Transform faults, and they are found where significant movement occurs along a fracture in the earth’s crust.

The San Andreas Fault
Clearly, the most famous and most visible transform fault in the world is the San Andreas Fault. This enormous fault stretches for over 1,000 miles from northern California, through western California, to the East Pacific Rise beneath the waters of the Gulf of California.

The Pacific Plate lies to the west of the San Andreas Fault and the North American Plate lies to the east. Scientists have determined that the Pacific Plate moves northwest at the rate of about two inches every year relative to the North American Plate. The western half of California lies on the Pacific Plate while the eastern half of California lies on the North American Plate. Besides the short term effects of being an earthquake ‘hot zone’, western California will, in about one million years, be part of Alaska (assuming Alaska stays put), as the Pacific Plate continues its northwesterly trek. Much crushing and grinding takes place as these two enormous plates move past each other. When sections of the plates become locked, stress builds up until the friction is relieved by a minor tremor, or major earthquake.

Mountain Ranges

While new ocean crust is constantly being created at mid-ocean ridges, old crust must either be destroyed or reduced at the same rate (or else the planet would be continually expanding and increasing in volume). The plates, therefore, emerging along mid-ocean ridges, sliding over the athenosphere, and grinding past other plates along transform faults, are almost all headed on a collision course. When two continents carried on converging plates ram into each other, they crumple and fold under the enormous pressure, creating great mountain ranges.

The highest mountain range in the world, the snow-capped Himalayas, is an example of a continent-to-continent collision. This immense mountain range began to form when two large landmasses, India and Eurasia, driven by tectonic plate movement, collided. Because both landmasses have about the same rock density, one plate could not be subducted under the other. The pressure of the colliding plates could only be relieved by thrusting skyward. The folding, bending, and twisting of the the collision zone formed the jagged Himalayan peaks. This string of towering peaks is still being thrust up as India, embedded in the Indo-Australian Plate, continues to crunch relentlessly into Tibet, on the southern edge of the Eurasian Plate.

Here's a more detailed chronological explanation.
About 220 million years ago, India was an island situated off the Australian coast, and separated from the Asian continent by a vast ocean called the Tethys Sea. When Pangaea broke apart about 200 million years ago, India began to move northward. Scientists have been able to reconstruct India's northward journey. When India rammed into Asia about 50 million years ago, its northward advance slowed. The collision and decrease in the rate of plate movement mark the beginning of the Himalayan uplift.

Fossilized Sea Shells near Himalayan Peaks?
When archaeologists found the fossilized remains of ancient sea-creatures near the peaks of the Himalayas they were, understandably, puzzled. Intriguing questions were raised. Was there once an ocean or other large body of water at the top of this enormous mountain range? Unlikely.

Had the entire planet, Himalayas and all, at some point in Earth’s long history, been submerged underwater? Possibly - but highly improbable. No theory could fully explain this apparent paradox. Until the theory of plate tectonics was put forth.

Briefly, it goes like this: As the Indo-Australian Plate, with India firmly embedded, approached the Eurasian continent 20 million years ago, its leading edge, comprised of oceanic crust, was first to be crumpled and uplifted. Slowly, the Himalayas rose and the leading oceanic crust of the Indian sub-continent, carrying the fossilized remains of its ancient ocean inhabitants, was thrust up by the crumpling crust in its wake. Thus, plate tectonics explains how the majestic peaks of one of the world’s great mountain ranges were once the deep sea-floors of an ancient drifting plate.

The European Alps have been formed in similar fashion, starting some 80 million years ago when the outlying continental fragments of the African Plate collided with the Eurasian Plate. Unyielding pressure between the two plates continues even today, resulting in the gradual closing up of the Mediterranean Sea.

Growing Mountains
As an underlying oceanic plate tips down, its ocean-floor sediment is scraped off along the front edge of the overriding continental plate. The result is an increase in the width and thickness of the overriding plate. This could be why the Andes, a long mountain range bordering the west coast of South America, appears to be growing higher. Perhaps sediment from the Nazca Plate, which is diving under South America in the Peru-Chile Trench, is scraping off on the roots of the Andes. This scraping adds thickness and buoyancy to the mountains so that they float upward more rapidly than their peaks can be eroded by wind and rain.

Mid-Ocean Ridges

Did you know that the Earth’s longest mountain range is underwater? The Mid-Ocean Ridge system, shown above snaking its way between the continents, is more than 56,000 kilometers (35,000 mi) long. This series of mountains and valleys marks where the Earth’s crustal plates are moving apart.

The driving force behind the process of plate tectonics is heat generated deep inside the earth’s core by radioactive decay. This heat reaches the surface primarily along the Mid-Ocean Ridge. One of the earth’s most dramatic topographical features, the Mid-Ocean Ridge is a continuous range of undersea mountains more than 12,000 feet high and 1,200 miles wide winding through 40,000 miles of the world’s oceans. It is here, at Mid-Ocean Ridges, that new sea-floor crust is produced and much of the earth’s internal heat is released.

At Mid-Ocean Ridges, two plates are pulling apart from each other as hot magma (liquid rock) emerges from the mantle and oozes forth as lava to fill the crack continuously created by plate separation. The lava cools and attaches itself to the trailing edge of each plate, forming new ocean floor crust in a process commonly known as sea-floor spreading.