The color of a gemstone is its most significant characteristic, and many jewelers consider it to be the most vital evaluation criterion. Gemstones are found in all the colors. Color of a gemstone depends on following three characteristics: hue, saturation, and tone.
Hue is the basic or unique color of the gemstone. It is described as the shade, tint or sensation of a color. While almost all gemstones have some shades of other colors, the most valuable gemstones are those that exhibit a pure color.
Saturation is a measure of the intensity or purity of a gem’s hue or color. A gemstone that is free of gray or brown hues is considered to be strongly saturated and is more valuable than a gemstone with lower saturation. Saturation often decides the cut of a gemstone. A high-quality gemstone cut delivers an even color throughout the stone and exposes the fewest inclusions.
Tone represents the depth of a gemstone color, ranging from colorless to black. In other words, tone is described as the relative lightness or darkness of a hue. Gemstone tone is described as “light,” “medium-light,” “medium,” “medium-dark,” and “dark.” Medium-light to medium-dark tone is considered as most valuable range.
All the above three characteristics are associated with each other. Each plays a crucial role in determining the gemstone’s color. The more intense the color, the higher the value.
Clarity is a term used to describe the absence or presence of flaws inside or on the surface of a gemstone. A flawless gemstone is rare and usually expensive. Most gemstones have inclusions, or tiny mineral flaws, that can be seen under magnification or by the careful eye. A gemstone may have inclusions, cracks, spots, clouds, or any other blemish or imperfection.
Clarity is a key factor in determining quality and the value of a gemstone. Inclusions not only distract the eye, but interfere with the behavior of light in the gem, and have a significant affect on brilliancy or sparkle. Generally most minerals contain inclusions and spots but if they do not affect the durability of colored gems then these inclusions or spots will not devalue much for the gems except diamond. Colored stones are classified into three ‘Types’, which are defined as under:
Type I colored stones include stones with very little or no inclusions. This category can include Aquamarine, Blue Topaz, Zircon, Morganite, Tanzanite, etc. Clarity in the Type I group is classified as VVS (minute to detectable), VS (minor), SI1 (noticeable), SI2 (obvious) or I (included)
Type II colored stones include stones that often have a few inclusions. This category can include corundum, garnets, iolite, peridot, quartz (amethyst, aitrine, ametrine), ruby, sapphire, spinel, etc. Clarity in the Type II group is classified as VVS (minor), VS (noticeable), SI1 (obvious), SI2 (prominent), or I (prominent, affecting appearance).
Type III colored stones includes stones that usually always have inclusions. This category can include emeralds, tourmaline, etc. Clarity in the Type III group is classified as VVS (noticeable), VS (Obvious), SI1 (prominent), SI2 (more prominent), or I1 (affecting appearance or durability).
A good cut is something that may give a gemstone beauty and brilliance. A gemstone’s cut refers to its proportions and symmetry. The stone should be symmetrical in all dimensions so that it will appear balanced and so its facets reflect light evenly. Even light reflection provides good brilliance. A well-cut faceted gemstone reflects light back evenly across its surface area when held face up. If the stone is too deep and narrow, surface area will be dark. If it is too shallow and wide, parts of the stone will be washed out and lifeless.
The process of cutting and polishing gems is called gem cutting or lapidary. A person who cuts and polishes gems is called a gem cutter or a lapidary (sometimes lapidarist.) Few gemstones such as pearls and coral (usually referred to organic minerals) are not cut at all and many times left in their natural state. However, it is customary to polish these items, as with all gemstones. The quality of a gemstone’s cut can have a dramatic impact on how it looks but only a small impact on the price per carat.
While cutting, color of a gemstone should also be taken into account for optical efficiency. If a stone’s color is highly saturated, a shallow cut will allow it to pass more light, while a deeper cut may increase the vividness of a less saturated gem. There is no generally accepted grading system for gemstone cut.
In the cabochon cut, the upper surface of the stone is smoothed and rounded into a simple curve of any degree of convexity; the lower surface may be concave, convex, or flat. All the remaining cuts have flat facets.
In the table cut, the facets of the natural octahedron are ground to smoothness and polished; one facet, the table, is ground much larger than any other and made the top of the gem, while the opposite facet, the culet, is left quite small.
The rose cut consists of a flat base and (usually) 24 triangular facets—resembling a cabochon with facets.
The brilliant cut is scientifically designed to bring out the maximum brilliancy of the stone. The crown of a brilliant consists of a table and 32 smaller facets, of which 8 are quadrilaterals and 24 are triangles; the base, a culet and 24 larger facets, of which 8 are quadrilaterals and 16 are triangles. The base and crown are separated by a girdle. The brilliant cut has certain proportions—in general, the depth of the crown is one third the depth of the stone and the width of the table one half the width of the stone.
In addition to the above defined cuts, stones are also cut in a variety of square, triangular, step, emerald, and trapezoidal faceted cuts. The use of such cuts is largely determined by the original shape of the stone. Large rubies, sapphires, and emeralds are often cut square or rectangular with a large table facet surrounded by a relatively small number of supplementary facets.
Similar to diamonds, a gemstone’s weight is also measured in carats where one carat equals 200 milligrams. However, in the case of gemstones, this may not give an accurate idea of its size. Different types of stones have different densities. Two gemstones of the same carat weight may be different sizes. For example, a 1 carat sapphire or ruby will be smaller than a 1 carat emerald, though they have the same carat weight, because sapphires and rubies are denser than emeralds. At the same time, a 1 carat diamond will be larger than a 1 carat ruby as a diamond is less dense than a ruby. Gemstones can also be measured in dimensions (diameter, length, and width).