CHATOYANCY IN CATS- EYE
Beauty is almost always associated with color and clarity (lean and transparent) i.e free from inclusions and defects. This is true for many valuable gemstones such as Diamond, Ruby, Sapphire and emeralds etc. But some gems owe their beauty to lack of transparency and abundant inclusions and defects.
Chatoyancy of Cat’s Eye effect
Some gemstones with acicular of needle like mineral inclusions or tubes oriented in one particular direction of the crystal structures create a silky reflection effect. When such stones are cut with a curved surface (‘en cabochon) they display chatoyancy of the Cat’s eye effect.
What is seen is a silvery streak of light, which is displayed across the curved surface. This has the striking resemblance to the pupil of a cat’s eye. You may have noticed that the pupil of a cat’s eye is round; however it becomes a sharp vertical line when the cat is directly exposed to the sunlight or under any form of artificial light.
But the gemstone Cat’s eye is seen well only under the sun of incandescent light, which is focused, but under florescence light the ray is diffused of dissipated.
This Silvery streak of light could be very sharp or diffused. And the effect could be either enhanced of diminished by the gem cutter by the variation of the curved surface.
The value of cat’s eye will depend on the effectiveness of the ray, the size of the stone and its colors. Some Common Colors in which cat’s eye occur are gray, greenish brown, apple green, greenish yellow, brown and honey color. Honey color stones are very much in demand.
Chatoyancy is also seen in many other gems such as Tourmaline, Zircon, Enstatite , Apatite , Fabrolite , Quartz, etc.
ASTERISM OR STAR EFFECT
An asterism is an optical phenomenon displayed by some gemstones in the shape of ‘star’ on the surface of a cabochon cut from the stone. As a single ray is displayed in some as cat’ s eye effect due to a single set of parallel inclusions, when there are more than one set of parallel inclusions in the gemstone it may show a four, six or twelve rays star depending on the mineral. These set of inclusion are set parallel of the crystallographic axes of the crystal. Like in the cat’s eye, the base of the cut stone should be made parallel to the layer of inclusions.
Stars of corundum group of gems are known as star sapphires(Blue, Yellow, violet) of star ruby if the stone is red and have six rays. Star sapphires and rubies get their asterism from the titanium dioxide impurities (rutile) present in them. Rarely sapphires have another 3 set of inclusions made of iron oxide causing six more weak rays, thus resulting in 12 ray stars.
Some gems such as garnet and diopside produce 4 ray stars, because they have only two sets of inclusions. Due to these inclusions, star stones are never transparent but either translucent or opaque.
A distinction can be made between two type of asterism: Epiasterism , such as that seen in sapphire gemstone. To see this effect, the stone must be illuminated from behind.
The presence of flakey inclusions in gemstones or glass creates reflections which are extremely attractive. In most cases inclusions are parallel to the base. Oligoclase feldspar contains mica flakes usually parallel and when the stone is cut en-cabochon with flakes parallel to the base, it produces attractive spangles, and the stone is then called ‘sun stone’. It has been imitated by using copper flakes in artificial glass and it is called ‘gold stone’. Heated amber when immersed in water causes cracks which are disorderly arranged and cause beautiful reflections and the effect is called ‘sun spangle.
The mineral called corundum forms as precipitous bipyramidal, barell shaped prismatic, flat tabular or rhombohedral crystals. It also occurs in massive and granular habits. Corundum is the mineral name which is used under the mineral groups. Most people are familiar with corundum, however very few know it by its mineral name.
Corundum is also a rock forming material. It for ms in silica-poor igneous rocks and metamorphic rocks, rich in aluminum. It is an insoluble material.
Most corundum are found in alluvial deposits as secondary gem deposits, are sources of rubies in several parts of the world. Ex. Sri Lanka, Myanmar, Cambodia, India, Afghanistan, Thailand etc. Corundum are found as a primary mineral in igneous rocks rocks such as syenite,
Nephline in locations where aluminous shale’s or bauxites have been exposed to contact metamorphism. Recent discovered corundum mineral deposit found in Sri Lanka in Kataragama area as blue Sapphire crystals.
Corundum is very hard mineral with a hardness of 9 on the mohs hardness scale. It is the 3rd hardest mineral known, with diamond and Moissanite being minerals with a greater hardness.
Physical properties of corundum are follows :
Luster - Adamantine, Vitreous, Pearly
Diaphaneity - Transparent, Translucent, Opaque
Colors - As Ruby, Padparascha & Sapphire
Streak - White
Cleavage -Non observed
Parting -Rhombohedral and Basel parting
Fracture -Irregular / Uneven, Conchoidal
Density -3.98-4.1 (Measured) 3.997g /cm3 (calculated)
Important optical property of corundum is refractive index- 1.762 to 1.770 and maximum Birefringence of 0.009Asterism is a optical effect in corundum. It shows s 06 rays or sometimes 12 rays which is rare. Those are called Star Sapphire.
A gem quality specimen of corundum with deep red color is known as 'Ruby
Ruby forms as bipyramidal, prismatic, flat tabular or Rhombohedral Crystals. It is red in color due to the presence of chromium. To appear as deep red ruby color chromium percentage should be 0.10 colors of Ruby very as pigeon Blood Red , Golden Red , Bright Red, These Color of Ruby are used as trade name
A gem quality corundum with Pinkish-Orange is called 'Padaparascaha'.
This Corundum variety mostly forms as bipyramidal and prismatic crystals. It is pinkish orange in color due to the presence of chromium and iron as impurities.
A gem quality corundum with any color or colorless is called Sapphire. Ex. Blue, Pink, Yellow, Violet, Purple, Green, Brown, etc.. and also Bi-Color Sapphire.
Different colors of corundums excluding ruby & Padparascha arecalled sapphires. Colors very as mentioned above.Impurities are Titanium, Iron, and Chromium.
Gemstone in rough from with the appearance of silky, Milky, Smoky etc.. is called Geuda. This Variety could be converted into Blue, Yellow etc..by heat treatment.
This is a special variety of corundum which shows dull appearance until convert into colorful gemstone by heat treatment. It looks like smoky, milky, silky snd some sre with diesel color inside the stones. Due to the presence of Rutile (Ti O2) as impurities, it appears as described above.
SRI LANKAN GEMS IN PICTURES
Sapphires for Royalty
Gem Mine - Sri Lanka
River Bed Mining
Collecting Gem Earth
Washing Gem Earth
Picking the Gems
Traditional Gem Cutting Maching
Chrysoberyl Cat's Eye
Fine Yellow Sapphire
TREATMENTS TO RUBY & SAPPHIRE
If you have eaten meat, fruit and milk products bought from a supermarket, you have consumed food which has been irradiated, dyed, heat treated (Pasteurized), coated with wax, injected with hormones and sprayed with dangerous chemicals. The food industry uses these treatment methods because most people prefer to buy appetizing, shiny, good size, bacteria-free food products.
Untreated food however, is available and some people are willing to pay higher prices for it. The organic produce often looks scrawny and dull, yet it's priced high. Vendors may point out the holes in the vegetables as an indication that no pesticides were used on them.
If the supply of gems were limited to those specimens that are naturally attractive, they'd be so expensive that most of us could never own them. Therefore, it's not surprising that the gem industry uses many of the same methods as the food industry to enhance the appearance of gems.
The heat treatment of corundum has become so common that all rubies and Sapphires are assumed to be treated unless otherwise indicated. Untreated stones are rare, and sell at a premium.
For centuries, Rubies & Sapphires have been heated to improved their color and clarity. However, in the past 20 years, heat treatment has been done on a wider scale and at much high temperatures - 1700C and above.
When corundum is heated at temperatures above 1700C the silk (fine hair like needles inclusions)is dissolved and produces color, thereby improving color and clarity.This high temperature heat treatment can turn silky off-white or near colorless Sapphires clear blue. Rubies having silk appear less brownish or purplish and improve their clarity when you heat corundum below 1600C and above 1200C, you can create or improve star corundumby causing silk to crystallize. Thus the heat treating process can go in two directions to improve thee silk and lighten the color.
Heat treating is widely accepted because it causes a permanent improvement of the entire stone.Nevertheless, high quality heat-treated stones are often valued less than their untreated counterparts.
Untreated rubies and sapphires are rare, and rarity is prized in the Jewelry trade.It doesn't matter whether a stone was heated or not. The overall quality determines the prices.
To detect heat treatment is rubies and sapphires, gemologists usually must examine them under magnification. Heated stones may have fuzzy color areas and bands, surface pock marks, melted facets. Dot-like rutile needles or glassy circular cracks around natural crystal inclusions. Fluorescent reactions to ultraviolet are also studied. Heat-treated Blue Sapphire, often turns a faint chalky green under short-wave U.V. light.
This treatment is usually done to make pale of colorless Sapphire without silk look blue. It may also be used to turn stones red, orange or yellow or to form a star. The pale stones are packed in chemical powders and then heated to 1700C and above until a thin layer of color covers their surface. Diffusion is relatively new (about 25 years old, according to patent records) and is not very well accepted by the trade. The color is permanent, but is only on the surface of the Stone. Consequently,the color can be polished or cut off leaving the grey or colorless interior exposed. It is important to deal with reliable sources and have major purchases checked by an independent gem laboratory.
Colorless sapphires are irradiated to make them improve color. Pink Sapphires may be irradiated to turn them to Padparaschas. Irradiation also occurs naturally. Some gems have been colored by natural radioactivity in the earth's crust.
OILING AND DYEING
Low quality rubies and sapphires (particularly cabochons and Indian star rubies and beads) are often dyed with a colored oil to hide cracks and improve color. To be oiled stones must have surface fractures which allow the oil to penetrate. Dyed corundum is generally not accepted in the trade. Dye treatments provide a practical means of making low-grade corundum look better. People who otherwise could not afford a natural ruby are able to buy one that looks acceptable.
SURFACE AND FRACTURE FILLING
It is not uncommon for rubies and sapphires to have pits or cavities, especially on its pavilion (bottom). Around 1984, rubies with glass-filled cavities began to be repaired. Since it was fairly easy to detect the types of filled stones, they were rejected by the world market and most have disappeared.
Since that time a new kind of filled ruby, which is much harder to detect such as borax as a result a glassy molten material is deposited in cavities and surface-reaching cracks. The glass infilling is relatively permanent and irreversible and it improves a stones durability since the fractures are healed shut.
Most of the in filled corundum on the markets today is ruby from the "Mong Hsu" depost is Myanmar (Burma), which was discovered around 1990. Unfortunately Mong Hsu rubies typically have numerous minute fractures. Very dense. "Silk" cloud and a strong purplish color which make most of them look like low grade cloudy rhodolite garnet - ordinary heat treatment turns the stones into attractive red color. Since "Mong Hsu" stones are heavily fractured they must be heated in borax flax to prevent cracking and to improve their clarity.
This is a new process in Thailand that involves the diffusion of beryllium into ruby and Sapphire at very high temperatures in an oxidizing atmosphere. It was first discovered in 2002.Beryllium diffusion can penetrate deep, sometimes coloring the entire stone. The colors produced by this process are mainly yellow, orange, yellowish orange, orange pink and orange red. Some types of dark Blue Sapphires can be lightened using beryllium-diffusion process and virtually any color of ruby and sapphire can be reproduced by this process.
Emerald belongs to the mineral species called beryl, which is a precious gemstone. In Sanskrit emerald is called Marketh and also Tarkshya. It is however, from its Persian name, Zamurrad, which traveled to Greek as Smaralds, then through Roman to Latin, as Esurde, and finally took shape as Emerald in the 16th century.
The emerald was called Neronianus, after the Roman emperor Nero using an eyeglass made of it for watching the feats of gladiators. Cleopatra's magnificent jewels included a profusion of emeralds from her own mines,Napoleon III was reputed to have presented Empress Eugenie an emerald 'Clover Leaf' sparkling with diamond dewdrops. Soon after her marriage, the city of Paris presented her with 2 wonderful diadems of emeralds as a wedding gift.
One of the finest emeralds known was said to be one of 136 carats, which used to belong to the Czars of Russia and is now among the Soviet treasures. However an emerald statute which was found in a tiny shop in a back street of Amritsar in August 1960 has made the Russian emerald seem small by comparison.
The Duke of Devon shire in England has an enormous crystal, which is a two inch cube with flaws and inclusions in the texyure.Purity, Weight, coolness, freedom from dust and beauty are the five principal qualities of emeralds, s according to the ancient belief "cleanse men from all sins"
Some of the world's finest emeralds come from the famous Mazo mines in Colombia. The Colombian emeralds are known here by the name of "Box Emeralds"they are compact and display fine color and water.
Egyptian emeralds are pale and often cloudy. Emeralds of Brazil are yellowish. Zimbabwe's Emeralds are in dark hue and black spots in its composition as other African Emeralds. All types of Emeralds are produced in the Urel Mountains they are less attractive.Emeralds are also produced in India,Pakistan,Norway, Australia, South Africa , Mozambique and Madagascar.
Emerald belongs to the hexagonal system of crystallization, as it comes under the species of Beryl. And is one of the lighter variety of transparent gem materials, its specific gravity is from 2.67 - 2.75, It is nearly double the size of a sapphire of equal weight. Chemical composition is Be3AL2Ai6O18 with Chromium (Cr+3) and thehardness of emerald is 7.5 - 8. It is transparent and its Iustre is vitreous. It possesses double refraction, though in very small degree, and acquires positive electricity by friction. There is another variety of Beryl also in green color due to vanadium. It is not called emerald, it is known as Green vanadium Beryl, and some of very fine green beryls due to nvanadium have been discovered. But to designate green beryl as emerald, it should show absorption lines in red in the spectrum.
Other Varieties Of Beryl
Can we talk abut precious gems without reference to ruby, emerald oralexandrite?all of them have two things in common ; they are beautiful and hance valuable and each has a fraction of chromium (Cr) in them. Cr produces the purest red in ruby and red spinel, the most baeutiful green in emeraldand a wonderful change of colour in alexandrite; green in day light and red in candle light or incandescent light.
Not only those, many other gem varieties get their beautiful green whaen a bit of cr is present in them, and most often refer to them with a prefix 'chrom'; chrom diopside, chrom tourmaline etc. Even without cr they are green but not the best green.
So would it not be interesting to know a little about chromium and appreciate its role as "king maker". Most Gemstones get their colour due to the presenceof transition elements in them: v, Cr, Mn. Fe, Mn, Fe Co, Ni, Cu.
In general ,electrons in atoms occupy certain orbital shells around the nucleus , like planets around the sun. Each shell represents an energy level. the innermost shell can contain 2 electrons, next shell upto 8, next upto 18 etc. The inner shells are full and generally outer shell contain a fewer electrons than it can accommodate. But this is diffferent in transition elements, inner shells may not contain full quota of electron (see figure 1-Cr). Cr can have upto 18 in third shell but it has only 12.
Electrons in most elements exist in these shells as pairs spinning in oppaosite directions. However in transition elements, in the incomplete shells many electrons will remain unpaired. When white light (dey light) falls on a gamstone, part of it is absorbed by the electrons and jump in to higher energy levels and the unabsorbad component of light reach us as the colour of the gemstone.
Remember light is a from of energy and white light consistsof different coloured waves as you see in the rainbow, each wave having a different colour and having a different colour and different amounts of energy. The amount of energy required by a paired electron is higher than that required by an unpaired electron. Therefore it is the unpaired electrons of transition elementswhich absord part of light and cause colours in them. Most of gems get their colour due to transition elements but there are some which do not have transition elements but their colouring mechanism is different.
Now let us look at the role of the chromium gems. It can exist as a an impurity or as a part of the chemical formula. in ruby, emerald and alexandrite, Crexists as a substitute atom for aluminium atom.. Cr atom has six unpaired electrons, three involving bonding with other elements in the atomic structure and the other three changing energy levels by absording energy from white light.
Three of the unpaired electrons in Cr can occupy in different orbits or energy levels. The amount of energy required to raise an electron from ground level A to C is about 2.25 electron-Volts or eV which corresponds to yellow-green light and hence some electrons absorb yellow-green range of light(See figure2).
The amount of energy required to raise electrons from A to D leve is 3 eV and it Corresponde to violet part of visible light . So some electrons absorb violet colour. Millions of electrons in the gemstone absorb yellow-green and violet colours, so that stone appears in red.
Emerald ( Be3 Ai2 Si6 O18 +Cr)
The force thet surrounds cr (which has replaced Aluminium) is weaker than that of ruby. Therefore the energy levele of electrons are different. in this case about 2eV of energy, it is red part of light that is absorbed and the residual colour is the most beautiful pure green.
Alexandrite ( BeAi2 O4+Cr)
The forces that surrounds Cr is intermediate in Strength as compared with ruby and emerald. Therefore the C level is between 2 eV and 2.25 eV. The slight variations in the absorption can cause different colours. In day light high energy blue-violet components of light is higher and therefore the stone is green in daylight. The candle light or tungsten light is rich in yellow red component and hence stone appear in red in that light.
In general , electrons in atoms occupy certain orbital shells around the nucleus,like planets around the sun. Each shell represents an energy level. The innermost shell can contain 2 electrons, next shell upto 8, next upto 18 etc. The inner shells are full and generally outer shell contain a fewer electrons than it can accommodate. But this is different in transition elements in transition elements,Inner shells may not contain full quota of electron (see figure 1-cr).Cr can have upto 18 in third shell but it has only 12.
Polyasterism is the display of more than one star within a narrow angle of observation; they aren't stars of same star-¬network. Although quartz and many other species display polyasterism the study emphasized the phenomenon in star sapphire concerning its high commercial potential.
About 0.1% of Sri Lankan star sapphire produce and greater percentage of fashioned stones display multiple stars or polyasterism. Relatively common two-stars are often called "Siamese twins" and rare three-stars "triplets" in the trade.They not only comprise 6-ray stars but rarely 12-ray stars too, and prevalent among blue star, star ruby, and all other sapphire colors, sometimes fetching higher prices than that of single star stones.
Apart from usual criteria. symmetry, of component stars dominates the valuation of polyasteric stones. Morphological disturbances in the single crystal or forming a compound crystal by bonding/fusing/cementing several crys¬tals togetherby geological reasons or Branching/penetrating of crystalsare the two main causes that convert a star sapphire into a polyasteric stone.
Morphological disturbances include - basal, prismatic and rhombohedral - glides, and are favored in the same precedence order if one type is restricted by local stress. strain or other reason.
Relative displacement of two or more sections by glide causes display of asterism independently in the individual sections arising from their rutile needle inclusions - this is the polyasterism in single crystals. Similarly compound crystal formed by aggregation of crystals display the effect.
Figure 7 shows relative displacement of two sections of kornerupine crystal by glide, which cuts the cat's-eye in half and shifts fire two halves by equal distance.Basal, prismatic and rhombohedral - glides takes place similarly in the sap¬phire stones.Figures-5, displays clockwise- polyasterism by basal glide (0001) ,prismatic glide (1120) , rhombohedral glide (1011) , and a photograph of a polyasteric sapphire caused by basal glide showing two stars.
Figure 6. Ways of forming a compound crystal of a polyasteric sapphire.(a) Three wire frame diagrams of sapphire crystal branching at the terminations displayed along with stones to be fashioned.(b) Polyasteric stone probably fashioned from a compound crystal shows group of rutile cores at the centre that have revolved by 30o from the background core yielding polyasterism. (c) Interpenetration from a side. (d) Bonding/ fusing/ cementing of two crystals together.