Where To Look

Bill James (Australia) writes:

The geologist is a man with a bunch of keys to nature’s jewel box. But there are many different keys for the box has many locks and most of them are hidden under the dust of ages. The written history of even the oldest and most celebrated jewels is only an infinitesimal part of their story. Their strangest adventures in the world of men are insignificant compared to the wonder of their creation.

Nearly all gemstones are minerals, some of which are rarer than others. This rarity is one of the qualities for which they are prized, for only a minor proportion of all the world’s gemstones exist in localities where they can be readily collected or mined. It is in reading the clues to these localities that the science of geology comes to the aid of gem seeker. It is as if nature had set up a great game of treasure trove for us with gemstones as one of the prizes.

If we were lucky, we may stumble upon a gem deposit by accident. But the odds of doing so are more on our side if we have an idea where to look. Like other minerals, most , but not all, gemstones were formed in association with igneous rocks. Igneous means formed by heat, and these are rocks which resulted from the cooling and solidifying of some of the molten matter making up the interior of the earth.

Many igneous rock are said to be of a crystalline texture. This means that their hard mass is composed of small crystals, the size of which is determined by the length of time it took the rock to cool. Other igneous rocks, such as obsidian, are glassy. According to the conditions under which cooling took place, igneous rocks are classed as either extrusive or intrusive. In other words, they either overflowed the existing surface or spread out beneath it.

Basalt is a typical extrusive rock. It is fine-grained because it cooled quickly on the surface. Granite is an intrusive or plutonic rock. Its texture is coarser than that of basalt because it cooled more slowly underground. At the depth of 40 miles inside the earth there are pockets of molten rock. In conditions of intense heat and pressure, these rocks stew up a superheated mineral soup known to geologists as magma.This magma tends to rise. If it succeeds in escaping through the earth’s crust by means of a volcano, it becomes lava. Otherwise it forms vast, dome-shaped subterranean masses of intrusive rock that are called batholiths and laccoliths according to their shape and size.

In the bygone ages, when the face of the world as we know it was taking shape, batholiths formed the roots and kernel of many high mountain ranges. The surface of a granite batholith many miles in area is exposed around Bathurst, New South Wales, Australia. Nowadays when a geologist finds granite or gabbro or any other intrusive rock on the surface, he knows that the forces of erosion have worn away the material that once covered it. He also knows that there is the possibility of ore deposits and gemstones not far away.

When the original batholith was formed, the magma also spread into cracks and fissures of the surrounding rocks. Geologists call the result a dyke. Because it is in conditions different from the parent magma, the material in the dyke becomes a different substance called a pegmatite. This very coarse-grained rock is one of the sources of ores and gemstones which occur as crystals of various shapes.

Owing to the intense pressures in which it is contained, the magma holds quantities of water, chlorine, fluorine and boron in superheated solutions. These substances are known as mineralisers. The mineralisers help to keep the magma liquid and allow a longer period in which the mineral crystals can grow. Sometimes the crystals attain great size. Beryl crystals up to several tons in weight have been mined from pegmatite dykes at Broken Hill, New South Wales, Australia.

Emeralds, aquamarines, topaz, tourmaline, zircon and large crystals of cassiterite (tinstone) are all found in pegmatite dykes. So is quartz in various crystalline forms. But not every dyke holds gemstones. It is one of the elements of nature’s hide-and-seek game than an exact combination of chemicals must be present at the right temperature. Most precious stones owe their value to microscopic traces of metallic compounds.

Quartz is sometimes injected into the surrounding rocks by itself as vein quartz. Sometimes, combined with ores of gold or other metals, it is in ‘pipes’. Cavities known to miners as vughs (pronounced ‘vuggs’) occur in the pipes. In these vughs, crystals of the various types of quartz gemstones grow from concentrated silica solutions. These also reach large size and one of the finest recorded in Australia was a 25 lb. rock crystal mined at Kingsgate, near Oban, in New England.

It takes a long time to grow a crystal of this size but it is nothing compared to the immense period over which nature has produced our gemstone resources. The oldest rocks in Australia, occurring in the south-west, date back at least 2000 million years. The forces that create gemstones were already in action then. At that time the earth’s crust was more subject to movement and change than it is now. It was an era of incredible violence. The rocks warped, buckled and broke, spewing out streams of glowing lava under skies darkened by incessant floods of rain and lurid with the smoke and flame of volcanoes. Monstrous upheavals and descents took place over large areas. During the last 1000 million years the Australian continent was split into islands several times by the movement of rocks.

About 135 million years ago, the highlands of the Queensland coast between Cape Melville and Rockhampton were parted from the rest of the continent as it is now. The sea also divided New South Wales and Victoria from Western Australia at this time. During Australia’s geological history there were at least nine periods of tremendous subterranean activity when rocks warped and folded and great batholiths formed. The first and second of these periods took place between 2000 million and 500 million years ago. Six times more the continent was reshaped over the next 300 million years, with the final episode occurring around 75 million years ago.

Most of Australia’s gemstone deposits trace to one or other of these periods. We can pick up stones that are older than the dinosaurs, stones that have endured while mountains higher than Kosciusko have risen and been washed away. Against the dramatics of earthquake and volcanic eruption, the steady stealthy activity of wind and rain, ice and running water has little impact. Nevertheless, these are the forces that have done more than anything else to shape the face of the land.

Where To Look: (continued)