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Gemstones2.Pdf The Science of Gemology Gemology can be defined as the science of gems. It is most closely Minerals as Gemstones allied with mineralogy, drawing on knowledge of chemistry, physics and geology. Knowledge of physics and chemistry aids in understanding the physical, chemical, and optical properties of minerals. Geological knowledge allows an understanding of the origins and probable natural sources of gemstones. Gemology is not only concerned with the study of natural gem materials, but also with gem testing and evaluation methods, the cutting, polishing and mounting of gem-quality specimens, as well as identification of the distinguishing properties and aesthetic value (grade) of synthetically manufactured gems. Gemologists must also concern themselves with the properties, value and sources of precious metals, as well as the market value of gems and precious metals. What constitutes a gemstone ? Gems traditionally held in highest esteem are diamonds, rubies, “Gemstone” is a collective term for all “stones” used for personal adornment (i.e. as jewelry) or for other ornamental uses. sapphires, emeralds, and some varieties of opal. However, large, high-quality specimens of semiprecious gems (e.g. Tourmaline) The term is usually restricted to “stones” of natural origin, however, often command a very high price even in their natural form. there are “synthetic gems”. Prepared precious gems Not all gemstones are minerals as such (e.g. amber, pearl, coral, jet, opal, ivory, petrified wood). Some are rocks (multimineralic). A “gem” can be thought of as a gemstone that has been cut and/or polished to accentuate its beauty. Gemstones are generally divided into two main categories: Precious gems: characterized by great beauty, durability, stability, large size and rarity. Large, colourful natural Semiprecious gems: Only one or two of these qualities are commonly tourmaline crystals (semi- represented. precious) can be very valuable. 1 2 Quantifying Beauty The value of most gems is based on three main qualities: As a general rule for coloured stones… 1. Beauty About 50-60% of a gemstone’s value resides in the quality of 2. Durability colour. 3. Stability About 20-30% of the gemstone’s value is judged by clarity (a measure of the absence of distracting or undesired impurities or inclusions and surface irregularities). About 10-20% of the value resides in the cut of the finished stone. There are several notable exceptions to this rule (e.g. the value of diamonds emphasizes clarity and cut; the value of turquoise is based on colour alone; opal’s value is largely based on the “play of colour” that it exhibits). Beauty Durability Durability is the resistance of the gemstone to damage. It is The term “beauty” is obviously dependent upon the properties of hardness and tenacity. subjective; the judgement of Hardness is a measure of the resistance of a gem to scratching. beauty of any given gem will Hardnesses of 7 and higher are though to be ideal, however, vary from person to person. only a few (10-15) known gemstones fall within this range. However, we can say that the beauty of a gem is based on the qualities of a single colour While very beautiful, ammolite or a combination of colours (or (made of the mineral aragonite) has a hardness of about 3.5-4, lack of colour, e.g. jet), lustre, so scores low in terms of This is a newly recognized transparency, or enhanced hardness. gemstone called “ammolite” optical properties due to (it is fossil mother of pearl from an cutting and fashioning. ancient squid-like organism). Tenacity is the resistance to bending, crushing and breaking It is valued for its beautiful colours. (through fracture or cleavage). 3 4 Hardness versus tenacity Other factors that dictate gem value Hardness is NOT the same as tenacity A number of additional factors will influence the value of a While diamond is extremely hard (cannot be scratched by any other gemstone. The three most significant ones are: mineral) it has a low tenacity due to its perfect cleavage (i.e. it can actually be broken along any of 4 cleavage planes). Weight Conversely, while nephrite jade is relatively soft for a gem (5.5-6; can even be scratched by dust in the air), it is composed of fibrous crystals Rarity that strongly resist tensile stress. Therefore, it has high tenacity. Demand/Fashion Natural diamond Nephrite jade Stability Weight Stability refers to a gem's ability to retain colour (i.e. its fade The value of a gemstone is also dependent on its resistance) over time in spite of exposure to heat, light (or other weight. radiation), or chemical assaults. More soluble minerals (e.g. carbonates) are likely to have lower The Carat (ct) is the standard unit of weight used stabilities, as are those that are more transparent to visible light. in the pricing of gemstones . For instance, a mineral such as calcite that dissolves readily in 1 carat is equivalent to 0.2 grams acid, would be of low value as a gemstone. Easier to remember: 5 carats = 1 gram. Due to differences in mineral density, or specific gravity, the visible size of one carat will vary from gem to gem. For example, a one carat sapphire (higher SG) will be much smaller than a one carat opal (lower SG). 100 carat diamond sold for Following on this, settings for particular US $8,000,000 gemstones are based on the stone’s dimensions in 2003 in millimeters, not the carat weight. SEM photo of calcite crystals etched by acid 5 6 Demand/Fashion The value of certain gemstones can also fluctuate with trends in fashion. For example, dark red garnets (called pyrope [“fiery”] Cullinan I garnet) from Bohemia became very popular in Victorian (The Great Star of Africa) times; buyers were willing to pay high prices for the privilege World’s largest diamond cut from 3,106 carat stone of owning them. After fad waned, the market price for these Largest cut stone at 530 carats garnets fell dramatically. Dimensions: 5.3 x 4.4 x 2.9 cm Discovered 1905, Premier Mines, South Africa, Named after Sir Thomas Cullinan, owner of the mine. Purchased by Transvaal Government for ~$800,000. Victorian Presented to King Edward VII as a birthday present from S.A. gov’t Bohemian Resides in the Tower of London, set in the sceptre of King Edward VII. garnet bracelet Current estimated value: over 200 million U.K. pounds sterling Natural pyrope garnet Mg2Al2(SiO4)3 Rarity How many minerals qualify as gemstones ? The rarity of certain gemstones, such as diamond, will Important gems greatly influence market value. Of the 4,000+ known minerals, only The rarity of gemstones reflects to some extent the approximately 70 possess uniqueness of natural geological processes that formed qualifications for them (just the right ingredients and just the right physical gemstones conditions), and also, the availability of gemstone bearing rocks at or near the Earth’s surface. Of these 70 minerals, only 15-20 are commonly Rare gemstones tend to command a higher price than encountered by the more common ones. average consumer for various reasons. However, when new discoveries introduce more gemstones of a particular variety to the global market, market price will tend to decrease for that variety. 7 8 Misconceptions on Naming The Deception of Colour: Example 2 Precious stones have been prized since the beginnings Even today, with modern identification techniques, common of humanity. citrine (yellow quartz) is sometimes sold as the more valuable gemstone topaz. Note different chemical But misinformation abounds regarding gems and their composition and hardness. mineral composition. This stems partly from the ancient practice of grouping precious stones by colour rather than mineral makeup (as discussed in the last lecture, colour can be deceiving in the identification of minerals). Citrine (Yellow Quartz) Topaz SiO2 Al2SiO4(F, OH) 3 Hardness: 7 Hardness: 8 Corundum The Deception of Colour: Example 1 On the other hand, a single mineral can be For example, rubies and red spinels are very similar in given different names when they are of gem colour, but are completely different minerals. quality. Classifying by colour led to the more common (and less A good example is the mineral corundum. valuable) spinels being sold to royalty as rubies (note different composition and hardness). Corundum is commonly dull in colour. Due to its great hardness (9 on Mohs scale of hardness), it is used widely as an abrasive. Specimens of corundum that have exceptional clarity and brilliant colouration find uses as gemstones. Ruby and sapphire are just unusually beautiful forms of a rather common Ruby (Corundum) Spinel (Fe,Mg)Al O mineral (corundum). (Al2O3) 2 4 Hardness: 9 Hardness: 8 9 10 Star Effect in Ruby and Sapphire Ruby Many of you will be familiar with the six-sided star pattern When corundum contains a sufficient quantity of chromium, it found in some sapphires and rubies. exhibits a deep red colour. The star pattern results from the presence of microscopic crystals (inclusions) of the mineral rutile (TiO2) Orientation of rutile crystals parallel to the six crystal faces of a corundum crystal causes a “play of light” that focuses light in a six-sided star. May be accentuated by cabochon cut. Natural ruby Cut ruby Star sapphire Star ruby Sapphire Beryl Like corundum, beryl (BeAl2 (Si2O18)). When corundum exhibits any colour other than red, it is is a mineral that displays different colours. called sapphire. The gemstone called emerald is a brilliant green variety of beryl Aquamarine is the light blue variety of beryl. Natural sapphire Cut sapphires “Common” beryl Emerald Aquamarine 11 12 Cutting and Polishing Brilliant (Diamond) Cut The brilliant cut accentuates a gemstone’s ability to Gemstones are cut and polished to accentuate their natural transmit and disperse light, and was principally developed for diamonds.
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