Gems were used as currency in many past civilizations.
Gems contain mineral structures that produce unique colors, surface lusters, hardness and transparencies. Not all collectible gems contain all of the necessary attributes to make them valuable, but many gems possess most of the features that set them aside from ordinary minerals. The appeal and possession of valuable gems dates back to the Persians, Greeks and Romans, who mined precious gems and learned cut and fashion them into decorative jewelry pieces.
Hardness
The hardness of a gem indicates one of its most important characteristics, making it desirable for fashioning and collecting. Hard gems resist surface scratches, which would mar them and quickly devalue their appearance. Hard gems also resist wear, giving them a longer life, especially when used in a setting, clasp, ring or broach. Hardness remains one of the most important features of diamonds, among other desired characteristics.
Color
Most gems can be recognized by the beauty of their natural colors. Gems with naturally occurring colors can be refined and polished to bring out the deeper hues inherent in their composition. The most prized colors coincide with the most sought-after gems, including the diamond (blue, white, yellow, pink), sapphire (blue, white, colorless, green, yellow, orange, brown, pink, purple, gray, black, multicolored), emerald (light green, dark green), ruby (red, slight orange to purples, some blacks and greys), garnet (red, reddish brown, green, dark red), turquoise (sky blue to green), peridot (olive green, bottle green), jade (green, blue, brown, white) and quartz crystal, which comes in a variety of colors and can be translucent.
Luster
The luster of a gem depends upon its surface texture in its raw form, but it mostly deals with the way the gem can be cut and polished. The hardness of a gem has a direct bearing on how smooth the luster appears on its surface, and this relates to the density of its molecules. Hard gems polish to a higher luster than porous or softer-surface gems, giving them higher reflectivity.
Rarity
Many gems have high value because of their scarcity in the natural environment, meaning that extreme measures are needed to find them. Such minerals include different colored sapphires, chrysoberyl's cat's-eye, jadeite, black opal, and pink topaz. Some gemstones have been recorded as nearly extinct, such alexandrite and demantoid garnet. Such gems can command from $250 to $5,000 per karat, with the exception of an alexandrite with good color change, which can go as high as 10,000 dollars for one-karat, according to Connoisseur Gems For Jewelry.
Classical Trends
Some gems rise to the top of popularity because of demand, reviews and public opinion. Some gemstones begin fashion trends and cults, as once happened with the popularity of turquoise in the 1960s. As demand increases with a limited supply, value escalates until it peaks. Gems which have gained popularity as rising stars include aquamarine, tsavorite garnet, tourmaline, imperial topaz and tanzanite.
Transparency
All gems have a natural crystalline structure which interacts with light in certain ways. Many of the most coveted gems have transparent or translucent qualities that make them more valuable than others. Certain gems have a refractive index figure, by which their optical properties are evaluated. The amount of internal luster, translucence and fluorescence contributes to the intrinsic value of many gems.
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