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The Secret Life of Rocks
Think they just lie around? Oh, no! Rocks are constantly meeting, mingling, and occasionally making gemstones.

By Martin Stone, 2003

 

 

Every so often you hear about a miner who literally trips over a rock, or takes a good. hard look at the ground in just the right place, and suddenly discovers a gemstone deposit. For the serious prospector, though, finding a deposit requires a good grounding in how the Earth is put together - the different types of rock you encounter and the conditions under which they formed. all of which determine whether or not gems could have grown there.

What are those prospecters looking for? Why are gems plentiful in some places and not in others? What is schist, and how does it happen? The answer begins at the center of the Earth.

Geologists agree that the planet is composed of three lavers - a central core, a middle area called the mantle, and a relatively thin crust upon which we live and where precious stones are formed.

The core is believed to consist of an inner hub of very dense rock, made up of about 80 percent iron plus various amounts of nickel, silicon, and cobalt. This is surrounded by an outer core layer of liquid rock, or magma, containing mainly silicates of iron, magnesium, aluminum, calcium, sodium, and potassium. The planet's middle layer, or mantle, is a semi-molten zone, and floating upon this is the crust, or surface. As a general rule, elements that dissolve easily in iron, like chromium and vanadium, tend to sink to the iron core, while the lighter silicates migrate up to the crust.

Those silicates, by far the most common minerals in the Earth's crust, form the basis for the majority of gemstones. The necessary elements combined in the heat and pressure of the mantle and then crystallized into minerals as they cooled in the crust.

Most of the commercially important gems, such as feldspar, quartz, tourmaline, beryl, topaz, and zircon, were created in the middle and lower levels -of the crust. Their slower rate of cooling as they transformed from magma to rock made it possible for large crystals to form. As the temperature of the magma dropped, individual minerals separated out in a process known as fractional crystallization. The feldspars were the first to solidify, producing large, well-formed crystals such as those seen in amazonite and adularia. As temperatures fell, one by one minerals separated and crystallized, with quartz being one of the last. That's why quartz crystals are usually smaller and less well-formed - except in geodes - since earlier crystallizations had already taken up most of the growing room.

Rock formed in this way is called igneous, or fiery, rock. Sometimes the rock will cool and harden underground and be transported to the surface via a volcano or other lava flow (magmatic rock), and sometimes the rock is composed of the lava itself.

The most common type of igneous rock is granite, which, because of its slow cooling rate, is coarse-grained, meaning that it tends to contain large crystals. Rock formed from lava flows, such as basalt, tends to be finegrained because it cooled quickly, and therefore only rarely contains gemsize crystals.

Igneous rock is host to pegmatites, some of the richest gem-bearing rock on the planet. Pegmatites consist of dikes or veins, and usually contain large, well-developed individual crystals. They formed toward the end of the cooling process, when residual liquid or gas - often containing some of the rarer elements - crystallized into veins in the host rock. It's those rare elements that were the key to forming gemstones, which otherwise are made from some of the most common elements on earth.

Occasionally a fresh magma flow will be forced into rock that has already cooled, causing interactions between the elements in the magma and the elements in the cooler rock. Gemstones such as alexandrite, ruby, and sapphire can be formed during these reactions.

It's often zones where a new stream of elements interacted with existing rock - be it part of the cooling and crystallization process, as in a pegmatite, or an instance where a magma flow brought new elements upward that gemstones form. Because those zones of interaction between the new rock and the old rock were so narrow, gems tended to form in narrow, concentrated veins rather than being scattered throughout the rock.

But that's just the beginning of the story. Rock that's exposed to the air is gradually broken down by the forces of water, wind, and temperature fluctuations, forming a new type of rock, sedimentary. Once the rock is broken down, water and wind combine to move the fragments about the surface, sometimes for long distances. Sedimentary rocks can be caught up in fastflowing streams and gradually settle into depressions in the riverbed, or in bends and bottlenecks. Being heavier, gemstones tend to settle in those beds, forming alluvials, or gem gravels.

Sometimes, however, layers of sediment get piled on layers of sediment, forcing the old layers downward. The new layers of soil put greater and greater pressure on the rock below it, ,until the pressure - combined with beat if the rock gets close enough to the boundary between the mantle and the crust - causes a chemical reformulation.

This new type of rock is called metamorphic. The key difference between metamorphic rock and igneous rock is that the metamorphic rock never quite reaches the molten state, whereas the igneous rock was molten to start.

The two types of metamorphic rock most likely have gemstones are schist which consists of a more or less even mixture of quartz, feldspar, and mica and gneiss (pronounced "nice"), which typically has light-colored layers of quartz/feldspar alternating with darkcolored biotite/amphibole.

Gemstones commonly found in metamorphic rock include ruby, sapphire, zircon, spinel, garnet. and jade, and sometimes it also plays host to gems usually found in igneous rock, like emerald and tourmaline.

Although we often think of rock as permanent and unchanging, in fact rocks are in a perpetual cycle of change. Igneous rock from beneath the surface is pushed upwards by volcanic action or tectonic actvity: once on the surface, it weathers and becomes sedimentary rock, only to be pushed gradually down into the crust to become metamorphic rock. As the rock changes. it carries gemstones with it. or sometimes the gemstones will form as a result of geological activity.

No matter what type of rock it forms in, each mineral crystallizes according to its own physical and chemical characteristics. Therefore, the variety of minerals found in a given deposit is limited by the local environment and conditions. Geologists know that if they find a certain mineral in a given locale, it is possible that other minerals with similar criteria for formation will be nearby, while minerals demanding an entirely different set of circumstances at birth will seldom be found in the same deposit.

For example, emerald is almost never found near ruby or sapphire because they require opposite conditions to form. Emerald is a beryllium aluminum silicate [Be3Al2(SiO3)6] whereas ruby and sapphire are types of corundum (Al203). Corundum generally forms in rock that is low in silicates, whereas emerald is a silicate.

Following that line of logic, you'd expect to find corundum in the same areas as other silica-deficient gemstones, like chrysoberyl and spinel. And in fact, in Myanmar (formerly Burma), it's very common to find spinel near the ruby and sapphire mines; likewise, you often see spinel and chrysoberyl in the sapphirerich alluvial gem gravels of Sri Lanka. While it's not always that straightforward a process, it works as a rule of thumb.

Knowing which minerals develop in company - or in association, as geologists say -helps the prospector identify the types of gems likely to be associated within his claim. Similarly, identification of a mineral can be aided by knowing which type of host rock it came from, For example, topaz and danburite crystals are so alike in color and appearance that a prospector might be easily confused unless he knows that they never occur together. Topaz is usually formed in igneous granitic rock, whereas danburite is mainly found in metamorphosed limestone. By identifying the host, the rock-hunter can identify the gem.

With advanced geological study, miners are learning more and more about where and how gemstones form, in ways that will eventually allow them to more accurately predict where deposits will be found. Until then, perhaps the luckiest miner is the one with a trained eye.

 

 
   
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