1)The per carat weight of a stone increases exponentially when the total weight of the gemstone increases.For example if the price of a 0.8-carat gemstone is 100 usd. Then a similar 3.5-carat gemstone of the same quality (similar physical properties like colour,transparency etc) might cost as high as 3500usd.
2)Gemstones that have extraordinary properties such as colour change under different types of light are more expensive than gemstones that do not have this property. Examples of gemstones that have this colour change property is alexandrite and some garnets. The colour change may be more prominent in some gemstones than others. The more prominent the colour change the higher will be the the price.Prices of gemstones that have the colour change properties can be even higher than rubies.
3)Treatment done on a gemstone also influences the value. Usually it is the totally untreated gemstone which has the most value.Examples of different types of treatment are heating,radiation,diffusion and oiling.
4)The colour of the gemstone also determines the price. eg among sapphires, various light tints of pink, yellow, mauve, green and white are referred to as "fancy sapphires". Generally fancy gemstones have much lower value than their counterparts.
5)Chrysoberyl cat's eye gemstone comes in different shades of honey, apple green, dark grey and brownish yellow. The valuation of chrysoberyl cats eye is similar to the valuation of star sapphires( see below). In cats eyes it is the clarity of the "streak of light" that determines its value.
6)Among star sapphires and star rubies, a simple classification into four categories will help the valuation. This classification is based on the clarity of the star and the colour of the background:
a) The star shows clearly in ordinary light with background colour that is either deep blue (star sapphire) or deep red (star ruby). These are regarded the best and command the highest prices in this variety.
b)The star is only visible in strong light but with a good background colour.
c)The star is visible in ordinary light but with a dull or weak background colour.
d)The star only shows clearly in strong light and there is an extremely weak background colour.
7) The presence of cracks, flaws and inclusions can decrease the value of a gemstone. In some gemstones such as rubies, the presence of small amount of inclusions is acceptable because it is proof that the gemstone is "Natural".
8)Rarity of the gemstone will be of value even if it is not commonly used in jewelry. Examples of rare gemstones found in Sri Lanka are: Ekanite, Sapphirine, Scapolite, Sinhalite and Taaffeite. These gemstones have "collector's value". Gemmologists, geologists and mineralogists all over the world collect such gemstones. Museums , Universities and research centers around the world also collect these gemstones.