Every stone used as an ornament, because of its beauty, durability and rarity, should only be called a gem. The beauty of a gem is determined by a set of factors such as color, transparency, brightness, special optical effects (color variation, light dispersion, opalescence); while durability is related to resistance to chemical and physical attacks. The rarity with which a stone occurs in nature is another important factor in determining its commercial value. However, tradition and fashion can decisively influence the final price. Thus, the diamond — which is not one of the rarest gems in nature — usually has a high market value because it is one of the oldest and most traditional stones for use in jewelry, that is: it never goes out of style.
The vast majority of gems are minerals, classified according to the following division: crystalline substances (diamond, topaz, amethyst, emerald, aquamarine); amorphous substances (such as opal and volcanic glass); organic substances (pearl, coral, amber) and rocks (lapis lazuli, turquoise and others). All these substances are natural. In addition to them, there are today on the market a large number of products partially or totally manufactured by man, trying to reproduce the brightness and beauty of these minerals. These are the synthetic gems: called coated, reconstituted or composite.
The denomination “precious stone” used to be used only for diamond, emerald, ruby and sapphire, as they were the most known and appreciated since antiquity; the others were popularly called semiprecious. “These terms are artificial and confusing, discrediting gems such as opal, aquamarine, chrysoberyl, amethyst or alexandrite, among other stones of great beauty, appreciated all over the world. Therefore, the distinction between precious and semi-precious stones should be avoided, using the term gem”, says gemologist Pedro Luiz Juchem, from the Geosciences Institute of the Federal University of Rio Grande do Sul (UFRGS).