Precious Gem Valuation by Appraisal Experts
Precious gemstones are graded in order to determine their value. All gemstones are graded according to the same system. Understanding gemstone grading is key to making good purchasing decisions. Review the grading guidelines before buying gemstones. Grading decisions are made by weighing all of the factors and deciding if the stone as a whole is excellent, very good, good, fair, or poor.
A gemstone’s grade, and thus value, is determined in large part by the stone’s color. When considering a gem’s color, buyers should keep in mind that there are three primary colors: blue, yellow, and red. All other colors are a combination of these primary colors. This means that most stones in secondary or tertiary colors will have strong, primary hues. This is why green stones are often described as yellow-green or blue-green. Many gemstones also include elements of white, gray, and black. In the traditional color system, these are considered neutral colors. When neutral colors are present in a stone, the stone must be evaluated in terms of saturation, hue, and tint. The chart below provides definition for these terms and for the color grading terms with which they are associated.
Saturation refers to the ratio of grays and other colors in a given stone. The higher the saturation, the less gray a stone contains. In general, stones with a very high saturation are much more valuable than stones with a high percentage of gray.
The word “hue” is often used in place of the word “color.” The hue of an emerald, for instance, is green. Gray, white, and black are not hues, meaning that they are not considered true colors. Some hues of gems are much more valuable than others. Such values tend to fluctuate over time and depend largely on the preferences of jewelers and buyers during a given era.
Any color mixed with white is described as a tint. In the majority of gemstones, the higher the tint, the less valuable the stone. Gems are considered more valuable when they are free of white. However, a handful of gems are considered more valuable when they are tinted.
The color grade of a stone describes the strength of the primary color. A stone that is 100 percent yellow would contain absolutely no other colors. Stones are generally assigned a higher overall grade if they have very pure coloring.
Color tone describes the amount of black or white mixed into a stone. This term is used to describe whether the stone is very light or very dark.
A gemstone is graded according to both color grade and color tone. Buyers who wish to buy the more valuable semi-precious gemstones should conduct independent research to determine ideal color grade and color tone values. Remember that it is not necessary to buy gems that are graded highly. Personal preference should almost always take precedence when buying semi-precious gemstones.
Color zoning is a term used to describe gemstones with colors that are located only in some parts of the stone. Zoning is considered undesirable. However, some zoning is more severe than other types of zoning. The chart below outlines the four different types of color zoning.
In stones without color zoning, the color is evenly distributed across the stone.
Faint color zoning generally refers to minor changes in the color saturation of a stone.
Gradual zoning refers to a blended weakening of the color.
Visible color zoning refers to stones that have only patches or layers of color. Colors may change abruptly in such stones.
A stone’s clarity refers to how much inclusion a gemstone contains. An inclusion is the presence of any material that is not a part of the mineral or gemstone. Many inclusions are colorless. While stones with some inclusion may be graded decently for having good character, stones that are too heavily included will not receive high grades. Gemstones are described as free of inclusions, very lightly included, lightly included, moderately included, heavily included, or excessively included.
Brilliancy is the measure of how much light a single stone reflects while in a stationary position. Stones with high brilliancy tend to command high values. How a stone is cut can have a significant effect on its brilliancy. Those stones that are very well cut generally have high brilliancy.
A stone’s depth is calculated by dividing its height by its minimum width. Depths are always described in terms of percentages. Most gemstone enthusiasts agree that 60 to 80 percent depth is ideal. Depth is important because it determines how the color saturation of a stone is perceived. Stones with too little depth may appear washed out while stones with too much depth may simply appear very dark.