Although not common knowledge to the public, most gemstones are considered in some way to improve color and clarity. No gemstone is a rarity. This is a common treatment and is widely accepted in the industry and are usually not de-value the stone.
Most retail jewelers do not divulge this information to the public to buy their own or they do not realize. If they are reputable and ethical foundation, they will be honest if queried on the subject. Do not expect this will provide the information without asking first.
Heat treatment is the most common method used to improve the color of Gems. Tanzanite, which has become very popular in the gem and jewelry to collect heat treatment is to get a beautiful cornflower blue color with lavender tones, and flashes red. Rough that must be heated to 600 degrees; others, Tanzanite as we know it will not be there.
Colored diamonds (which does not occur naturally) is irradiated to obtain the most sought after red and blue. Natural colored diamonds are very expensive and rare. The "Hope" diamond is an example of natural blue diamonds. Natural colored diamonds can come in a rainbow colors like blue, brown, yellow, orange, red, purple, pink, and green, but only for the cost is too high for most people to get. Irradiated diamonds create a beautiful color in a more affordable cost. Natural colored diamonds are radiated in the earth's crust that took place millions of years. One who knows how to speed up the process and produce the same color in the laboratory. Champagne diamonds and yellow are more in the nature and usually not irradiated to get their color.
Topaz is a gem that needs treatment to get the colors is known as "London Blue" color that lasts about 18 months to be processed. A popular color is called "Mystic Fire" is a rainbow of colors in the same gem. Other popular color is pink. Pink topaz is much cheaper than pink sapphire. Pink is one of the most popular color in gemstone jewelry. The "Mystic Fire" colors have been created by a process called bulk diffusion. Topaz is a natural white (also called silver) or gold-colored, known as "valuable" or "imperial" topaz is rare and most expensive of topaz. Because the request that other colors such as Swiss Blue, London Blue, Pink, Mango, Neptune Blue, Mystic Fire, etc. white topaz (which is needed to make colored topaz) has become increasingly scarce and rise in costs.
There are a number of gemstones that do not need and they take various enhancement treatment. Colors that occur naturally right from the earth. They only need to be polished and cut to show their beauty. The gems that come to mind that garnets, chrome diopside, spinel red, and Peridot.
No more rare gemstones and therefore in May to get a higher price than treated gemstones. As a collector or investor, I will explore a variety of non-enhanced, if available and affordable.