GEMSTONES AND THEIR INDUSTRIAL USES

By Allan Taylor

Reproduced from the Field Geology Club of SA Bulletin, Vol. 34, No.7, pp 7-9, August 2005

Introduction

Gemstones may be broadly grouped into "Precious" and Semiprecious" types according to their rarity, beauty and hence value.

They were desired for their use as ornamentation (by the elite, religious and royalty etc) for the status they emanated for the owner and wearer. Also, they were a convenient means of transporting wealth in times of war (less weight than gold bars). From ancient times the precious category was limited to diamond, ruby, sapphire and emerald. All other gemstones were considered to be semiprecious or not so valuable, including the common ornamental stones such as agates, jasper, malachite, hematite.

Thus gemstones became important items of trade. Some were responsible for the development and exploitation of new lands e.g., diamonds in South Africa and Brazil, emeralds in South America, opals in Australia. The idea that gemstones could have other important uses was slow to develop and did not blossom until the end of the 19th century.

The seed of the idea was to do with abrasives. Early civilizations widely used quartz sands as an abrasive to saw blocks of limestone and other soft rocks. The Mayan Indians of Central America (ca. 600 AD) sought after the red garnet sands from rivers to use as an abrasive to saw and fashion the tough local jadeite into beautiful ornaments. Diamond was the hardest substance known but due to its (then) rarity its use as an abrasive was limited to specialty engraving purposes.

Mineral Synthesis

During the Middle Ages when the science of chemistry was developing, many alchemists were attracted to the idea of turning base metals (especially lead) into gold. In principle this was a good idea but it turned out to be a "red-herring". However, lateral thinking prevailed (pre- de Bono) and attention was directed to making the rare precious stones -- diamond, ruby and emerald, which if successful would save the expense of importation from India and South America.

The synthesis of these precious stones is broadly a three stage process. Firstly one must recognize their chemical composition. By the early 1800's it was known that diamond was pure carbon, ruby and sapphire were slightly impure alumina and emerald was of more complex composition containing beryllia, alumina and silica. The second stage is to synthesize in the laboratory the gem material from commonly available and cheap starting materials, e.g., diamond from carbon, ruby from alumina and emerald from common beryl, the source of beryllia.

Thus the latter part of the 19th century saw the blossoming of the science of mineral synthesis and the beginning of the art and science of growing large single crystals in the laboratory. The successful conversion of these starting materials by a patented or secret chemical process (involving different temperatures, pressures and fluxes) would result in obtaining the desired gem material, but always in a micro-crystalline form, which was identified by microscopic examination. The final stage is to grow this product as large crystals for gemstone use. The initial most spectacular success was with the growing of ruby and sapphire crystals to a size and quality not found in nature.

By 1910 in Europe an industry had developed around the Verneuil flame fusion process to grow large crystals or "boules" of ruby and sapphire (coloured corundum, composition Al2O3) and later also of spinel (composition MgAl2O4) in various colours. Production of emerald proved more difficult. Here the same melt process did not work because on cooling only a glass resulted because of its high silica content (ca. 67 wt%). Not until 1939 was the synthesis of emeralds of gemstone size and quality achieved by Carroll Chatham of San Francisco using a molten flux process at ca. 800║ C. Now there are several producers of synthetic emerald using this process and the manmade product can be found in most jewellery shops today, labeled as "created emerald".

Synthesizing diamond was more difficult because of the very high pressures required for its stability, about 100 kbars. Various patents were taken out in the 1960's and 70's by General Electric Corporation in the USA for processes to grow diamonds. Microcrystalline synthetic diamonds have the form of perfect octahedra and proved to be more abrasive than the natural material. This is because the octahedral face is the hardest part of a diamond and all other faces, or parts, are not so hard, as hardness varies with crystallographic direction. Natural low grade industrial diamond or crushed natural diamond dust has fewer octahedral faces exposed and is therefore a less efficient abrasive than the synthetic material.

Today the drilling of oil wells is dependent on drill bits studded with diamond crystals and the building and ornamental stone industries use saws and special endless ropes containing diamonds for cutting up stone blocks. The lapidary uses diamond loaded saws, copper laps impregnated with diamond dust and diamond polishing powder, especially to fashion the very hard or tough gemstones such as sapphire and jade. But how is it possible to grind and polish a diamond, it being the hardest known substance, you may wonder? The answer is very slowly using diamond dust on high speed iron laps. The secret is not to have any facets coinciding with an octahedral face or plane, which is not polishable.

Crystal Growing

The real breakthrough in crystal growing was the flame fusion process invented and developed by the French chemist Auguste Verneuil in 1902. An oxy-hydrogen flame is directed downwards into a furnace and alumina powder is dropped through the flame, where it melts (m.p. ca 2000║ C). The molten material on falling cools and crystallizes as a single crystal forming initially a ball (or "boule") and if slowly lowered will form a rod up to a foot long. Addition of a little chromium gives a red ruby colour and a mixture of iron and titanium gives a blue sapphire colour. A greater colour range could be obtained by growing spinel (MgAl2O4). These boules were sliced up with a diamond saw and fashioned into synthetic gemstones. Synthetic ruby was also used for making hard bearings in watches and other mechanical instruments.

Up until 1940 nearly all the manufacture of this synthetic gem material was done in factories in Switzerland, France and Germany. The Allies were caught napping with a supply shortage when World War II commenced. However, the USA rapidly developed an expertise in crystal growing. Prominent in the field was the Linde Division of Union Carbide Corporation, a producer of industrial gases, which established a factory in East Chicago and a crystal R & D laboratory at Speedway, Indianapolis. As a sideline the Linde Division produced synthetic "Star ruby and sapphire" similar to the natural star gemstones by heat treating and exsolving titania needles within the crystals, causing the 6-ray star effect in cabochon cut stones.

During the 1960's scientists directed their attention to the fluorescence properties of crystals, particularly ruby, but all other fluorescent crystals were of interest. Ruby has the property of absorbing short wave ultraviolet radiation and emitting the energy at longer wavelengths (red), due to electronic shifts in the chromium atoms incorporated within the crystals. It is best observed in the pure synthetic crystals because in natural ruby (ditto for emerald), the ubiquitous presence of iron tends to quench the effect.

Ruby laser rod and colourless sapphire made by the Verneuil Process (1962) and ruby laser pointer

The "ruby laser" was invented in 1960. This devise produces an intense coherent red light beam with minimal divergence. An alternative method of growing ruby was developed giving more perfect crystals. Called "crystal pulling", it works by lowering a seed crystal into a melt and slowly raising it as a single crystal rod, a method that is used for many other important crystals, including metallic silicon, a semiconductor of great interest to physicists. Silicon metal is made by fusing pure quartz sand with coke which on release of carbon dioxide gives metallic silicon. As a semiconductor it gave birth to the transistor and allowed the development of microelectronics, integrated circuits and the "silicon chip" which seems to have no end in useful applications.

The ruby laser is now familiar to most people because of its widespread applications. It is designed into every CD player and long distance telephone operations. It is used in surveying and microsurgery to burn out unwanted tissue and in depilation. High energy lasers will cut though steel plate and drill holes through diamonds. Many lecturers today use a tiny ruby laser pointer that operates from a 4.5 volt battery.

All this crystal growing activity originated from an early desire to grow the precious stones ruby, diamond and emerald. The subsequent huge growth of R & D activity has gone the full cycle with the production of many wonderful synthetic crystals, some becoming new gemstones. This is particularly the case with diamond simulants, the look-a-likes for diamond. Every jewellery shop window has rings set with "cubic zirconia" having similar optical properties but not as hard as diamond. Not so common are strontium titanate, YAG (yttrium aluminium garnet), lithium niobate and rutile which all look like diamond when facetted.

Time is running out! Consult your watch and no doubt it will have "quartz" written on the dial. If so it has a wafer of synthetic quartz crystal whose piezoelectric property controls the time keeping to great accuracy and operates by a silicon chip. If it's a Rolex it will have a "watch glass" made of colourless synthetic sapphire which is scratch resistant. íViva las piedras preciosas!

The End