Full Astrology > Gems & Stones

The essential qualities of a Gem Stone

It becomes relevant to consider the qualities that make gem materials suitable for use in jewellery.

The first and most obvious of these qualities is Beauty. Unlike a gem's more tangible properties, its beauty cannot easily be quantified as it depends in the main on subjective factors to do with its appearance. If the stone is a transparent colored gem, the depth of color and degree of transparency will be the prime factor. However, in the case of a gem such as a diamond, beauty will be determined by its brilliancy, fire, and optical purity and, in general the absence of any body color. With precious opal, the quality of its iridescent play of color will be the deciding factor

Rarity is another quality which must be present in some degree in all gemstones worthy of the name (colored glass, while it may be quite beautiful in terms of hue and transparent, is by no means rare) unlike beauty, however the rarity of gem can be affected by factor such as supply and demand, and by both fashion and the scarcity of the source material. Amber and pearls have become popular again and therefore more expensive Brazilian sources in the eighteenth century.

Unusual optical properties add to a gem's rarity factor as with alexandrite and cat's eye chrysoberyl. Diamonds are expensive, but as the total world production of all types of rough diamonds for 1987-1988 was in excess of 80 million carats, the cost of the finished product is hardly attributed to its rarity. In this case, De Beers' virtual control of the supply side of the sourly/demand equation, and the economics of the mining, polishing and marketing of the gem play a dominant part.

The third essential quality, which must be present in a gem, before it can be considered suitable for use in jewellery, is its Durability. This is a more practical quality than either beauty or rarity, but without it is a gemstone would not be able to survive either the day-to-day wear and tear experienced by a piece of jewellery, or the chemical attack from pollutions in the atmosphere, and would soon lose its surface polish.

Durability, which includes the property of hardness and toughness, is therefore a most important quality in a gemstone from the wearer’s point of view. Equally important is the influence of a gem's hardness on the work of the lapidary and the diamond polisher.

Organic and Inorganic gems

Jewellery has from the earliest times included gem materials which have an organic as well as mineral content, and because of this the science of gemology today covers not only mineralogy, geology, optics and chemistry, but also overlaps into the field of zoology, biology and botany, among the gem materials used in jewellery, the largest group are those of the mineral kingdom. Gems such as ivory, bone, pearl, coral, tortoiseshell, jet and amber are of organic origin.

The geological formation and occurrence of gemstones

The Earth is the third innermost planet of the star we call the sun. Its structure, as indicated by studying seismic wave records of earthquakes, is made up of a central core, a mantle and a crust, the core has a diameter of about 4300 miles (7000 kilometers) and is thought to be composed of two parts, the outer core is about 1300 miles (2100 kilometers) thick and appears to consist of a liquid. Rock magma beneath this lays the solid inner core having a diameter of around 1700 miles (2700 kilometers), and is believed to consist of very dense rocks composed of 80% iron and varying amounts of nickel, silicon and cobalt.

Between the core and the crust is the mantle, an intermediate semi-molten zone having a thickness of about 1800 miles (2900 kilometers). Above the mantle floats the solidified crust of the continents, whose thickness varies from a mere 3 miles (5 kilometers) under the oceans to 45 miles (72 kilometers) deep "roots" thrusting down beneath the higher mountains. Of these three structural zones it is the earth's crust, which is of prime interest to us, as this is the birthplace of our gem minerals and its geology, is the key to their occurrence.

Grading of a Diamond
A diamond's cost is based on the characteristics known as the "4 C's". Color, Clarity, Cut (proportion) are the quality elements, which together with the Carat Weight determine the value of a stone. The closer a diamond grades to the left of one or all of these scales the rare and the more costly it will be. While clarity is frequently assumed to be the most important factor of all the "C's", in fact, color and cut (especially cut) have a more profound affect on the visual appearance of a diamond.

Color

Ideally, a diamond should have no color at all, like a drop of spring water. Diamonds of known color are used as comparison stones for color grading. Grading is done by comparing, the diamond to be graded against "master stones" under either artificial or natural daylight. A machine called the "Colorimeter" can be used for color grading but there is no substitute for the trained human eye.

Diamonds are found in all colors, from clear colorless, transparent stones to ink black ones. Colors of high saturation such as red and green are very rare and command world record prices. Collectively, these colored diamonds are called "fancy colored" diamonds, and include the fancy colored yellow diamonds found at the far end of the color spectrum of the so-called "white or colorless" stones will be focus on here.

The best color for a colorless diamond is, in fact, an absence of color. However these stones are relatively rare, the most frequently found diamonds (~80%) being of industrial quality. Those suitable for gem use (~20%) usually have some yellow tint, due to the presence of a small amount of contaminating nitrogen. As one would predict, a truly colorless stone will carry a premium price, and the larger that diamond, the greater the premium per carat.

Diamonds are graded for color only as loose, unmounted stones in the inverted position (table down, pavilion up), and under very specific conditions of lighting and background, and also the distance and angle of the observer in relation to the stone. A color scale has been devised by the Gem Trade Lab. of the Gemological Institute of America (GIA) divided alphabetically from D (the absence of color) to Z (fancy yellow color), and is recognized world-wide. Each letter grade represents a small range of color, and not just a single color. Diamonds are color graded by being compared to a set of master stones. Another important and well-respected grading lab is the American Gem Society Labs, which uses a similar grading system, but a numerical scale from 0 to 10 (See figure below).

Effect of color on price

In general, as the amount of yellow in the stone increases, the value of a diamond decreases, that is until the fancy color grades are reached, and then the value goes up again. Roughly, for an internally flawless (IF) round brilliant diamond of 1-1.5ct, prices fall about 25% in going from D to E color, and then about 10% more for each additional grade (F=-35%, G=-45, H=-55%) until one gets to H color, where the difference decreases to about 5% less going from H to I color (I=-60%). So the first color decrease from D to E will provide you with by far the largest increment in saving money. These numbers are only rough estimates, and not meant to use as a pricing guide.

Factors Affecting Color

Factors that can affect color include fluorescence, color enhancements such as high pressure, high temperature treatment (HPHT) and irradiation.

Fluorescence, produced by ultraviolet light from the sun, by black lighting or other long-wavelength UV source, occurs in an estimated 35% of gem grade diamonds. (Virtually all diamonds fluoresce when exposed to X-rays, and this forms the basis for their identification and collection at mining sites). The UV light excites electrons in the diamond crystal, which then release this absorbed energy in the form of visible light, producing a blue, or sometimes other color, of faint to very strong intensity. Once the light source is removed however, the fluorescence is no longer observed. If, in rare situations, light emission continues for a period after the exciting light has been turned off, and this phenomenon is called phosphorescence.

High Pressure/High Temperature (HPHT) treatment , is a process developed by General Electric whereby type IIa diamonds of low color (N-O) or even fancy brown color, can be converted to colorless/near colorless (D-H) forms by an annealing process involving high pressure and temperature. There is some concern that these color-enhanced stones may come to market undetected, but new research has shown that they can be largely detected using expensive and sophisticated equipment. Similarly, several companies have also applied the HPHT technique to type Ia brown diamonds, yielding fancy yellowish green and greenish yellow colors. Pink and blue colors have also been produced by the same technique.

Irradiation of diamonds using a nuclear reactor or linear accelerator, sometimes in combination with heating, is used to produce a variety of different fancy colored stones.

Clarity

A diamond's clarity is determined by, the number, nature, position, size and color of internal characteristics called "inclusions" and surface features called "blemishes". These irregularities occurred in the liquid magna (volcanic rock) within which the diamond was created. Diamonds is pure form of carbon, however, during crystallization other minerals nearby, or even other bits of carbon forming more quickly may have trapped within the cooling mass. These show themselves as the various characteristics, which make up the clarity of a diamond (included crystals, feathers, clouds etc). Clarity is measured on a scale ranging from pure (flawless) to heavily included (I-3). The clarity of a diamond is graded by using 10X magnification under good lighting by an experienced grader. The final clarity grade is usually determined by how easy the inclusions and blemishes are for the grader to see.

GIA Clarity Grading Scale
Clarity Grade	Definition
Flawless (Fl)	Free of all inclusions and blemishes.
Internally Flawless (IF)	No inclusions and only minor blemishes visible at 10x magnification.
Very Very Slightly Included (VVS1)	Minute inclusions extremely difficult to locate at 10x.
Very Very Slightly Included (VVS2)	Minute inclusions extremely difficult to locate at 10x.
Very Slightly Included (VS1)	Minor inclusions difficult to locate at 10x.
Very Slightly Included (VS2)	Minor inclusions somewhat easy to locate at 10x.
Slightly Included (SI1)	Noticeable inclusions easy to locate at 10x.
Slightly Included (Sl2)	Noticeable inclusions very easy to locate at 10x
Imperfect (I1)	Obvious inclusions usually easy to locate with the unaided eye.
Imperfect (I2)	Obvious inclusions easy to locate with the unaided eye.
Imperfect (I3)	Obvious inclusions very easy to locate with the unaided eye and which may, threaten stone's durability.

Clarity characteristics are an inherent part of a diamond's life, and can arise from events, which occurred during its formation deep in the earth, the mining procedures used to collect it, the cutting of rough into its final shape and the wearing of the stone.

On most grading reports both blemishes and inclusions are plotted for the face-up and pavilion views of the stone. In general, blemishes are plotted in green, inclusions are plotted in red and extra facets and some naturals are plotted in black. Often, to avoid a messy looking plot, pinpoints, clouds and other minor details of clarity are not plotted, but indicated under "Comments:" at the bottom of the report.

Blemishes
Abrasions: tiny nicks on facet junctions or the culet, caused by wear or coming in contact with other diamonds.

Extra Facets: small facets placed to remove imperfections; not part of the cutting style. (Not to be confused with Added Facets which are added symmetrically and are part of the cutting style.)

Naturals: part of the original crystal surface remaining on the polished stone, frequently in the area of the girdle.

Polish Lines and Marks: tiny parallel lines or surface clouding left by irregular polishing or excessive heating during polishing, respectively.

Rough Girdle: a grainy or pitted girdle surface usually caused by poor workmanship.

Surface Graining: structural irregularities in crystal growth; may appear as faint lines, causing grooved or wavy surfaces and often cross facet junctions.

Inclusions
Bearding: tiny feathers extending inward from a bruted girdle surface. (Bruting is the process of rubbing two diamonds together to achieve the rounded shape of the diamond).

Cavities and Chips: large/deep openings, and small/shallow openings in the diamond's surface, respectively.

Clouds: hazy or milky areas of many very small, usually crystalline inclusions.

Feathers: cleavages or fractures often white and feathery in appearance. (There are 4 cleavage planes in diamond, which run in octahedral directions. Fractures are breaks along planes other than cleavage planes and may alternate with them to form step-like feathers.)

Included Crystals: mineral crystals, such as garnet or peridot, contained inside a diamond.

Indented Naturals : natural rough surfaces, that penetrate the stone and may distort the girdle outline.

Internal Graining: regions of irregular crystal growth that may appear as milky or colored lines or streaks, or may be reflective.

Laser Drill Holes: a tiny tube made by a laser; the surface opening may resemble a pit, while the tube usually resembles a needle.

Needles: needle-shaped included crystals.

Pinpoints: areas of minute, dot-like inclusions.

Twinning Wisps: cloudy areas produced by distorted crystal growth.

 

Cut

Cut, sometimes the forgotten "C", ensures that a given stone has maximum brilliance and sparkle, which would not be the case, were the stone cut for weight alone. We use the following scale to grade a stone on its overall appearance. The proportion shows angles and percentages for round brilliant cut diamonds; but angles and percentages are for diamond cutters and graders. Simply put, when looking at a diamond, if it doesn't catch your eye or if it doesn't flash in the light, it's probably not well cut. Good cutting is what brings fire to the eye. N ature is responsible for the color and the clarity of a diamond. The cut, that aspect that unlocks a diamond's hidden beauty, is the skill and craft of a talented diamond cutter.

The awe-striking beauty of a cut diamond emanates from its inherent physical and optical properties in combination with a cutter's ability to maximize the display of these properties. It is the precision, care and artistry, which the cutter brings to the stone that releases the diamond's full potential. In purchasing a diamond one must consider two elements of cutting, which are of prime importance - proportion and finish (which is further sub-divided into two categories, polishandsymmetry).

Here we focus on the modern round brilliant cut diamond because it is the most popular shape sold and it produces the best light return, in the form of brilliance, dispersion and scintillation, to the eye.

Brilliance is the total light reflected to the eye from both surface and internal reflections.

Dispersion is the separation of white light into its spectral (rainbow) colors and is caused by its refraction, which bends each wavelength of light a little differently to cause separation into individual colors.

Scintillation is the flashing or sparkling of light which occurs when either the diamond or light source are moved.

To easily understand what goes into evaluating the cut proportion and finish of a standard round brilliant (SRB) diamond, let's first take a look at a diagram showing each element of the finished product (above). Down the left-hand side are listed the major structural features of the diamond, and down the right-hand side the different facet names.

There are a total of 58 facets, called the table (1) and culet (1), bezel (8), star (8), upper girdle (16), lower girdle (16) and pavilion (8) facets. Often there is no culet facet and the stone is pointed on the bottom, leaving a total of 57 facets. Also, in higher quality goods the girdle is frequently faceted, but these facets are not counted in the total.

Finish , which includes polish and symmetry, is also an important contributor to a diamond's beauty.

Polish features largely describe the condition of the surface of the stone, and include abrasions of the facet junctions and the culet, rough and bearded girdles, nicks and pits, and scratches and polishing marks if they do not affect the clarity grade. You will remember some these elements were also listed as blemishes in the clarity-grading scheme. However, if the blemishes are not serious enough to affect the clarity grade, they are included under polish as part of the finish grade.

Symmetry characteristics include many features of poor and/or careless workmanship such as an off-center table or culet, a table not a regular octagon shape, an out-of-round or wavy girdle, non-parallel girdle and table, facets which don't meet or point properly or are misaligned between the crown and pavilion, and naturals and extra facets which are not graded under clarity.

Proportion - The full realization of the potential of a diamond's brilliance, dispersion and scintillation and getting the proper balance between these three elements is achieved mainly through proper cut proportions. The important proportions are:

1. Table size
2. Crown height and angle (angle of the bezel facets with the girdle)
3. Pavilion depth and angle (angle of the pavilion facets with the girdle)
4. Girdle thickness
5. Culet size

As shown in the diagram below, if a round brilliant diamond is cut too deep (nailhead) or too shallow (fisheye) light leaks out of the pavilion and does not return to the observer's eye. In diamonds with too steep a crown angle (>38deg.), exiting light is directed out to the side where it isn't visible, or it can be reflected back into the stone. In diamonds with large tables, if the crown angle is too shallow (<30deg.) there is little visible dispersion of light .

Carat Weight

This is the easiest "C" to understand. The weight of a diamond is directly related to its size, and, although incorrect, these terms are sometimes used interchangeably. The unit of weight for diamonds and other gemstones is called the carat. This is a term, which comes from the ancient use of the very uniformly sized carob seed in measuring small weights, which is found in tropical climates. These seeds were used until this century to weigh precious gems.. A table of common weight-related terms is given below. One carat is subdivided into 100 "points". Therefore a diamond measuring 75 points is 3/4 carat in weight, or 0.75ct. There are five carats in a gram.

Carat Weight	Equivalent Weight
One Carat	200 milligrams = 0.200 grams (There are 5 carats in 1 gram and 142 carats in 1 ounce.) A carat is divided into 100 parts each called a point. So 1 carat = 100 points.
Three-Quarter Carat	75 points
Half Carat	50 points
Quarter Carat	25 points
One Grain or "a one grainer"	25 points or 1/4ct.; A diamond of approximately 1ct. would be called a 4 grainer, one ~ 1.5ct. Would be a 6 grainer, and a 0.5ct. Stone a 2 grainer.
Melee	Diamonds weighing less than 1/5ct. (less than 20 points). Stones this size are set using various techniques as groupings in jewelry.

Effect of carat weight on price: Because diamonds are more rare the larger they are, the carat weight is not directly proportional to the total asking price. For example, the total price of a 2ct., D, IF, stone is not twice the total price of an identical 1ct. stone, but is more than three times the price, and an identically graded 3ct. diamond is about 7.7-times as expensive as the 1ct. stone. Also, a premium is added to diamond prices as they reach and exceed each 1/4ct. increment in weight.