Gemstones are minerals, mineraloids, or non-minerals that can be faceted, polished, or otherwise modified in shape to enhance their appearance. In addition to being attractive due to its color, or the ability to reflect or transmit light, a gemstone typically is fairly hard and durable. Gemstones have been sought after and treasured throughout history. They have been found in ruins dating several thousand years. They are valued as gifts symbolizing love.

Generally, the price of any gemstone is determined by the "4 C's" - color, clarity, cut, and carat weight. The most valuable gemstones are commonly referred to as "precious" gemstones, are extremely hard. Diamonds, emeralds (beryl), rubies, and sapphires (red and all other colors of gem-quality corundum) fall into this category. Softer, less durable minerals such as jade (nephrite and jadeite), garnet, amethyst , citrine, tiger eye, rose quartz (quartz), peridot (olivine), tourmaline, and turquoise usually are less valuable and are classified as "semi-precious". 

In recent times, gemstones have been divided into two groups: diamonds and colored stones. This distinction can be somewhat misleading, due to the fact that many diamonds are colored and many "colored stones" are not. One major difference between the diamond and colored stone markets is that the diamond market is largely controlled by one organization -- De Beers Consolidated Mines. Through their near-monolithic control of diamond mining and distribution, this company has done much to create a fairly stable market for diamonds, and relatively small diamonds are readily available in a wide variety of qualities. In contrast, most colored stones are mined with more primitive methods, by much smaller companies, and supplies are much more variable. Many colored stones are much rarer than diamonds of comparable size and quality and are often unavailable. A sizeable deposit of a stone may be discovered and quickly distributed to the market, only to become scarce again in a couple of years. Such uneven supply and less regulated distribution often contribute to wide price variations.

The following general rules apply to all gemstones:

1. Vivid, saturated colors are more highly prized than subdued or grayed-out colors. Deeper colors are more highly prized than lighter ones, unless the depth of color is so great as to make the stone appear blackish. The best color for any gemstone should be obvious from several feet or even several yards away. For example, a ruby should be intensely red from across a room, and a blue sapphire should be obviously blue, not black. The exception to the rule occurs when the extremes are desired -- truly colorless diamonds are valued more highly than those with pale colors, and a truly black diamond would be worth more than one that is merely dark gray.
2. Larger stones are more sought after than small ones, although stones too large for use in jewelry tend to have lower per carat prices.
3. Gems with fewer and smaller inclusions are generally more sought after than those with more numerous and larger inclusions, unless the inclusions contribute in a positive manner to the appearance of the stone. For example, insect inclusions increase the value of amber. Fine inclusions that cause star or cat's eye effects increase the value of stones such as corundum or chrysoberyl. Quartz containing large included crystals of rutile or tourmaline is often more valuable than quartz without.
4. Rarer stones are more highly prized than more common varieties. However, if the stone is so rare that it is essentially unknown to the general public, its value suffers and it is relegated to the status of a "collector stone." Stones such as boracite, childrenite, ekanite, eosphorite, painite, and simpsonite are extremely rare, attractive, and durable, but they are unlikely to command prices appropriate to their rarity.
5. Well cut stones of good symmetry, attractive design, and fine polish are more prized than poorly faceted stones.
6. Stones of famous provenance are more prized than those lacking in personal history.
7. Pairs or suites of stones matched for color, clarity, and cut are more highly valued per carat than single stones, especially if the stones are rare on an individual basis.
8. Stones that have been enhanced in color or clarity by artificial means are worth considerably less than unaltered stones of the same appearance. Many gemstones are altered to enhance their appearance. This may be done to produce colors not usually found in nature, to improve color, to improve clarity, to reduce porosity and stabilize color (by preventing absorption of discoloring oils and other substances), or to enhance durability. Depending on the stone and the treatment, such alteration may be easy or impossible to detect.

• Irradiation: It is common to irradiate Aquamarine, London Blue Topaz, Emerald, and Diamond as well as other stones. This brings out color and removes imperfections. Colorless topaz is irradiated in large quantities and then heat treated to produce various shades of blue. Yellowish diamonds are often irradiated to produce a wide variety of colors. Other stones, such as tourmaline, are sometimes irradiated to enhance or produce new colors. In many cases, the effects of irradiation are somewhat unstable and can be reversed by heating.
• Heat Treatment: Many gems are routinely heated under controlled conditions to improve color (aquamarine, sapphire, ruby, tourmaline), alter color (sapphire, amethyst to citrine, topaz, zircon), or improve clarity (sapphire, ruby). Since natural heating also occurs (e.g., in volcanic areas), the artificial effects are sometimes indistinguishable from natural effects. In most cases, the results of heat treatment are permanent.
• Dye: This is the most common treatment used. On clear stones, dye may be visible in cracks that are darker than the rest of the stone. Sometimes dye appears as a residue that rubs off or white patches. Lapis and Rose Quartz are commonly dyed. Amethyst and citrine are often dyed. Black Onyx is permanently dyed in normal processing.
• Coating: Jasper is often dipped in petroleum products to bring out color and to seal it. Emerald is oiled and turquoise is waxed.

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