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What makes a valuable mineral as personal adornment are basically two characteristics, beauty and rarity. But that does not mean that the rarer gem is always the most valuable. Citrus and amethyst, for example, both varieties of quartz, have different prices; the amethyst, although more common, is more valuable.

On the other hand, there may be some paradoxes. Andalusite, for example, is a relatively rare gem and is therefore little known. Being little known, it is little sought after and, with little demand, it turns out to be relatively cheap.

Another factor to consider is fashion. There are times when a particular gem is most sought, while others fall into relative oblivion. And, finally, there is the question of local abundance or scarcity: amethyst and agate are much cheaper in Brazil, the world's largest producer, than in Europe.

The diamond, in many ways the most important of the gems, has its value greatly influenced by an extra factor: the production and sale of this gem are largely controlled by a single company, DeBeers Consolited Mines, which controls the supply and very influences the final price. This influence has already been greater and the trend is further diminished by the increasing presence of synthetic diamonds in the gemstone market.

The value of a particular gemstone depends on four factors:

- Size: One 1-carat (200 mg) gem, for example, will always be worth more than two-half carat with the same quality. It should also be remembered that the gems have different densities (the opal is much lighter than the topaz), so gems of the same size can have different weights.

- Color: in principle, the darker the color, the more valuable the gem. The green tourmaline is an exception; and the diamond, unless it has a well-defined color, is all the more valuable the more colorless it is. It is also important that the color is uniform.

- Purity: the absence of inclusions (impurities and fractures) is always desirable. Emeralds, however, are only pure in very small buds, for it is normal that they are full of fractures, filled with impurities.

- Stoning: Good color and good purity gem may have its price reduced if it is not well cut. This is particularly important in the case of diamond, since, for the most part, it is colorless, it has an important feature in the gloss. And a good gloss depends a lot on a good lapidation.

All this makes it very difficult to draw up a list of the most valuable gems, unless one considers purely the market value. So, it is enough to see the current quotation in specialized companies, remembering, however, that the values ​​change according to several variables, such as those said above, predictable to a greater or lesser degree.

Taking into account technical and market criteria and using the highest average price per carat (1 carat = 200 mg) paid in the international market, the ten most valuable gems today are as follows:

1st Diamond: up to $ 63,000 per carat. Amount likely to be influenced by the increasing presence of synthetic diamonds on the market.

2nd Tourmaline paraiba: up to US $ 15,000 per carat. Initially discovered in Paraíba (hence its name), it was later discovered also in Africa. The Brazilian deposits, however, are already depleted and the Africans are in the process of being exhausted, which should raise this price (if not increased). Value so high is explained by the incomparable blue color of these gems.

3rd, 4th Ruby and sapphire: up to $ 12,000 per carat. Ruby and sapphire are different varieties of the same mineral, the corundum. The ruby ​​is red and the sapphire can have any other color, being more valuable to blue.

5th, 6th, 7th Emerald, opal-black and alexandrite: up to $ 9,000 per carat. The price of the emerald varies greatly because of the impurities it may have; Pure gemstones are always small. Opal-black is one that has a dark background, on which are emphasized its numerous colors. Alexandrite, besides being very rare, stands out for having green color in natural light and red in artificial light.

8th Demantoid: up to $ 5,000. The demantoid is a rare green garnet.

9th Cat's Eye: up to $ 3,500. Variety of chrysoberyl, as well as alexandrite, the cat's eye gets its name by displaying chatoyance, luminous strip that gives it the appearance of a feline eye. Although this feature is also present in other gems, none of them reaches such a high price.

10th Topaz-imperial: up to $ 2,000. Produced only in Brazil, the imperial topaz has orange, pink, salmon or reddish color. Of them, the most valued is red.