Here are the factors that we consider in determining gemstone value.
Gemstone Species and Variety
The gemstone species is a scientific classification based on the chemical composition of the stone. A given species of stone may have several varieties. The different varieties share the same general chemical formula with only minor differences due to impurities within the crystal structure. But these minor differences can make a huge difference in the color of the stone. Some varieties of stone are common while others are rare (and more expensive).
The example in Table 1 below is for the mineral beryl [ Be3Al2(SiO3)6]. The different varieties of beryl have vastly different values (all other quality factors being equal) based on color, which is caused by impurities in the crystal lattice of the stone. We all know that emerald is much more valuable than aquamarine, but both are essentially the same stone, the only difference being the impurities that were present during their formation. As little as 0.01% impurities in the crystal structure can change the color.
As the size of the stone increases the value increases much more than the ratio of the sizes. This is simply because it is more unusual to find larger gemstone material than smaller material. This relationship is highly dependent on the variety of stone and in some cases even dependent upon the actual source of the rough material.
The clarity of a gemstone is defined by the number, size, distribution, and type of inclusions (sometimes called flaws) in the stone. There are many types of inclusions, and rarely they may actually increase the value of the stone. Most often the transparency of the stone is diminished by inclusions, and the reflection of light, called brilliance, is negatively impacted. In some circumstances the durability of the gemstone is affected by its inclusions.
There are many methods for describing gemstone clarity, but probably the best known is that used in the diamond industry, originated by GIA (Gemological Institute of America). RSA Gems uses the following scale to assess clarity of its rough and cut gems.
- IF – Internally Flawless. This means that any inclusions in the stone are invisible to the human eye, even under 10x magnification. For most stone varieties this is an extremely rare designation.
- VVS – Very, very slightly included. Stones with this designation may have one or two very small inclusions that can be seen with a 10x loupe, but cannot be seen with the naked eye. This designation is often called “eye-clean.”
- VS – Very slightly included. One or two very small inclusions that can barely be seen by the naked eye, but can be seen readily under 10x magnification.
- SI1 – Slightly Included 1. Some localized inclusions (not spread throughout the stone) that can be seen by the naked eye. These inclusions are slight enough to not significantly detract from the beauty of the stone, nor do these inclusions pose a hazard to the durability of the stone (for example, no small cracks penetrate the surface of the stone).
- SI2 – Slightly Included 2. One or more obvious inclusions that may negatively impact the beauty of the stone. Very small inclusions may penetrate the surface of the stone or be spread throughout the stone.