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Gemstone Basic Characteristics Defined
COLOR:
Color is usually the first characteristic that is recognized. Research has shown that there is a definite color preference ranking for what stone colors garner the most attention. Yellow has the highest attention value. People see yellow objects more quickly than that of any other color. Brown and green are low attention colors. Other high preference colors are blue, followed by red, purple and green. Yet despite these facts, the diamond, usually colorless, gets the highest preference rating amongst all gemstones.
STREAK:
Streak is related to color, different because the color of the mineral may differ from the color of the streak. Streak is the color of the powder of a mineral. Two minerals that appear to have the same color may have different colors when in a powdered form. Streaks can be tested for by rubbing a mineral across a tile of white unglazed porcelain and examining the color of the resulting streak.
LUSTER:
Luster refers to the way light interacts with the surface of a crystal. It does not describe color or shape, but is related only to transparency, surface conditions, crystal habit and the index of refraction.
These are several examples:
- Adamantine refers to very "gemmy" crystals
- Dull means there is no reflection off the surface
- Greasy refers to a surface that looks like it has oil on it.
- Resinous describes a look like that of resins (e.g., dried glue, or chewing gum)
INDEX OF REFRACTION:
The index of refraction is the geometric ratio of the angle at which light comes to the crystal (called the angle of incidence) by the angle at which light is bent as it enters a crystal (called the angle of refraction). Metallic minerals do not have an index of refraction because they do not allow light to enter the crystal in the first place.
BIREFRINGENCE:
The difference between the highest and lowest index of refraction in a mineral is called the birefringence.
HARDNESS:
Hardness is one measure of the strength of the structure of the mineral relative to the strength of its chemical bonds. The Mohs Hardness Scale starts with talc at 1, ends with diamond at 10. The higher the number, the harder the mineral.
CLEAVAGE:
When a mineral breaks it does so either by fracturing or by cleaving. Crystal cleavage is a smooth break that results in a flat crystal face. First cleavage is reproducible (a crystal can be repeatedly along the same parallel plane). The same mineral will always have the same cleavage.
FRACTURE:
Fracture refers to how a mineral breaks. The resulting surface is very recognizable. A fracture can be called smooth, irregular, jagged, and splintery, just to name a few.
CRYSTAL SYSTEM:
Every mineral is comprised of distinct crystals. Crystal system refers to the shape of the crystals. Minerals don't form their own unique crystals, but rather they form crystals consistent with the symmetry of the class of which the mineral is a part.
TECHNICAL CRYSTAL HABITS:
Crystal habits refer to the shapes and aggregates of a particular mineral. This can often be the most important factor in identifying a mineral. Most minerals have different forms that are peculiar to only to one, or just several.
SYMMETRY:
Symmetry refers to the way a crystal repeats the facets or faces on the crystal's surfaces. The symmetry of the crystal impacts the index of refraction.
TRANSPARENCY:
Transparency (diaphanity) refers to the way light interacts with the surface of a substance. There are three kinds.
- "Transparent" means that light can enter and exit the surface of the substance in relatively undisturbed fashion,
- "Translucent" means light can enter and exit the surface of the substance, but in a disturbed and "Opaque" means light can't penetrate the surface of the substance at all.
SPECIFIC GRAVITY:
Specific gravity is the density of a mineral divided by the density of water. A mineral's specific gravity corresponds to its mineral's its density expressed in grams per cubic centimeter.
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