Semi-Precious Gemstones
Traditionally, common gemstones were classified into precious stones (cardinal gems) and semi-precious stones. The category of precious gemstones was largely determined by a history of ecclesiastical, devotional or ceremonial use and rarity. Only five types of gemstones were considered precious: diamond, ruby, sapphire, emerald, and amethyst.

In current usage by gemologists, all gems are considered precious, although four of the five original "cardinal gems" are usually — but not always — the most valuable. Rare or unusual gemstones, generally meant to include those gemstones which occur so infrequently in gem quality that they are scarcely known except to connoisseurs, include andalusite, axinite, cassiterite, clinohumite and iolite. Gems' costs vary from $500 to $2000(US Money). It depends on the type of gem, and how rare it is, such as the expensive diamond.

Treatments applied to gemstones
Gemstones are often treated to enhance the color or clarity of the stone. Depending on the type and extent of treatment, they can affect the value of the stone. Some treatments are used widely and accepted in practice because the resulting gem is stable, while others are not accepted most commonly because the gem color is unstable and may revert to the original tone.

Heat:  Many gemstones are treated by heat, either to improve color or clarity. Most Citrine is made by heating Amethyst and melting amethyst and citrine together makes ametrine. Much Aquamarine is heat treated to remove yellow tones and give a purer blue. Nearly all Tanzanite is heated to low temperatures to remove brown undertones and give a more desirable blue/purple color. A considerable portion of all sapphire and ruby is treated with high heat to improve both color and clarity.

Radiation: Most blue topaz, both the lighter and the darker blue shades such as "London" blue, has been irradiated to change the color from white to blue. Some improperly handled gems which do not pass through normal legal channels may have a slight residual radiation, though strong requirements on imported stones are in place to insure public safety. Most greened quartz (Oro Verde) is also irradiated to achieve the yellow-green color.

Waxing/oiling: Emeralds contain natural fissures that are sometimes filled with wax or oil to disguise them. This wax or oil is also colored to make the emerald appear of better color as well as clarity. Turquoise is also commonly treated.