What is a gem?
A gem is generally defined as any mineral that is highly prized for its beauty, durability, and rarity, is used for personal adornment, and has been enhanced in some manner by altering its shape usually by cutting and polishing. A wider definition includes a few rocks, such as obsidian, and a few organic substances, such as amber (a fossilized resin). By far the majority of gems, however, are cut from the crystals of minerals. precious metals are not considered to be gems, nor are items carved from minerals but not used for personal adornment, such as figurines, bowls, or vases.
The first quality a gem must possess is that of beauty . This is subjective: some may prize a gem’s interplay of light and color, while others may first be drawn to gem’s intricate cut. With an almost endless combination of color, shape, and fire (play of light), gemstones are capable of a range of aesthetic styles.
Hardness or toughness in a gem is desirable quality, suggesting enduring value. Some gems require care to prolong their longevity. certain gems resist chipping or scratching, but fade after long exposure to direct light; dry environments may cause some to crack, while others are susceptible to damage from acids .
A gem may be rare for a number of reasons. The gem material itself may be rare, such as emerald, or a more common materials may exhibit an unusual color or clarity. some particularly soft or fragile stones are rare in cut form, as they require the work of highly skilled lapidaries.
However gem interacts with light is the very essence of its nature. Light is the source of a gem stone’s beauty, color and sparkle; it is also a useful tool for the identification of gems, as each stone has its own particular set of optical properties. For example, there are a dozen or more red gemstones and many of the red hues within each type of stone will have many different shades. all these properties are a way of identifying gemstones, although no single one is diagnostic in itself. some categories such as lustre are subjective observations; others, such as a mineral’s reflective index, are objective. A gemologist identifying a stone will use a number of different methods and instruments to narrow the possibilities. examination of one or all of the optical properties may be needed.
One of the most desirable qualities in a gemstone is beauty, and an important part of this is the stone’s color. in gems, color is caused when light is absorbed within the crystal, or refracted-changing direction as it passes through the gem. White light is composed of many colors; when one or more of those colors is absorbed, the remaining light emerging from the gem is colored. this can be brought about by the presence of trace elements that cause certain wavelengths to be absorbed, or can be a part of the gem’s chemical structure.
A gem’s lustre is the general appearance of its surface in reflected light. There are two basic types of lustre: metallic and non-metallic. Precious metals have metallic lustre, and gemstones non-metallic, with the exception of a few like hematite and pyrite. lustre that relate to gems include vitreous, waxy, pearly, silky, resinous, greasy, earthy, metallic, and adamantine .
When light passes through a transparent gem, it changes speed and direction. This is called refraction. The change in the speed of light as it passes from the air into a gem is called the refractive index. the change in directed, or bending, of the light can be used to calculate the gem’s Rl. Diamond’s high Rl results in flashes of light seen when the gem is moved its “Fire”. the greater the dispersion of white light, the greater the fire.
The study of the emission of light according to its wavelength is called spectroscopy. Devices known as spectroscopes are used to measure light waves as they pass through gemstones the spectroscope has a small slit for light to pass through. when a gem is placed between a light source and the slit, a light spectrum is produced. dark bands appear where certain wavelengths are absorbed by the stone. these bands are characteristic of various elements, enabling identification of the gem’s chemical make-up. the three spectra one the right reveal much about the gem’s composition.
When the need arises to identify or value a gem, it is sent to a professional gemologist who is certified by a professional gemological body. the gemologist will examine the gemstone for various physical and optical properties in order to make an identification and evaluate its quality. An essential quality of a gemstone is durability. A gemstone’s physical properties determine how durable it is, and how susceptible to cracking.
One of the principal determinants of durability is the gem’s hardness, or the relative ease or difficulty with which it can be scratched. measured on the Mohs scale, hardness does not equate to strength, as very hard minerals can also be quite brittle. gemstones below 5 on the scale are too soft foe wear and even stones of 6 or 7 will scratch and abrade.
Specific gravity (SG) is a measure of the density of substance relative to that of water, and determines how dense a gemstone is. It is measured as the ratio of the mass of the substance and the mass of an equal volume of water – so a mineral with an SG of 2 is twice as heavy as water. Specific gravity can be determined using specialized balances or liquids that allow minerals below a given SG to float and those above it to sink. However exports can often gauge the specific gravity of a gem purely by its heft.
Cleavage is the property of a mineral that causes it to break along its atomic layers, where the forces bonding its atoms are the weakest. some gems have cleavages in several directions, some of which may be very easy to trigger, meaning that the gem can be easily broken if sharply knocked. because they follow the atomic planes, cleavage surfaces are often smooth.
Fracture is another way of describing how a mineral breaks. In fracture, however, breakage takes place across the mineral’s atomic planes, rather than along them, as it does in cleavage, because there are no obvious planes of fracture. Distinctive fracture may help with identification.
Streak is the color of the powdered mineral, determined by rubbing a piece of the mineral across a piece of unglazed porcelain, leaving a streak of color this can used for identification, particularly with minerals that occur in different colors.
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