Uma pedra colorida, pedra preciosa ou gema, (também às vezes chamada de pedra preciosa ou semipreciosa, uma bela jóia ou joia), é um pedaço de mineral que, em forma cortada e polida, é usado para fazer joias ou outros adornos. Certas rochas (como lápis-lazúli), ou materiais orgânicos que não são minerais (such as amber or coral), are also used for jewelry, and are therefore often considered to be gemstones as well.
Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone.
The traditional classification in the West, which goes back to the Ancient Greeks, begins with a distinction between precious and semi-precious stones. In modern usage the precious stones are diamond, rubi, sapphire and emerald. This distinction reflects the rarity of the stones in ancient times, as well as their quality. All are translucent with fine color in their purest forms (except for the colorless diamond). These stones all also very hard (diamond being the hardest at 10 on the Mohrs scale, with ruby and sapphire rating an 8).
The traditional distinction does not necessarily reflect modern values, por exemplo, while garnets are relatively inexpensive, a green garnet called Tsavorite can be far more valuable than a mid-quality emerald.
Use of the terms ‘precious’ and ‘semi-precious’ in a commercial context is, arguably, misleading in that it deceptively implies certain stones are intrinsically more valuable than others, which is not the case.
In modern times gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology, specific to the field of gemology. The first characteristic a gemologist uses to identify a gemstone is its chemical composition. Por exemplo, diamonds are made of carbon (C) and rubies and sapphires of aluminum oxide (Al2O) – ruby being the red variety of the species corundum, while any other color of corundum is considered to be sapphire.
Gemstones are classified into different groups, species and varieties. Por exemplo, the emerald (verde), AQUAMARINE (azul), red beryl (vermelho), goshenite (colorless), heliodor (amarelo), and morganite (rosa) are all varieties of the mineral species beryl.
Gems are characterized in terms of refractive index, dispersion, specific gravity, dureza, cleavage, fracture and luster. Material or flaws within a stone may be present as inclusions.