Precious vs. Semi-Precious Gemstones
The rarest gems are classified as precious stones. These include diamonds, sapphires, emeralds, tanzanite, and rubies. Precious stones are more expensive than semi-precious but they follow the same color, cut, and clarity rules as diamonds for value. All other gemstones are classified as semi-precious. However, today these separations do not have much significance. It is more common to simply classify between diamonds and colored stones.
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Colored Stone Grading: Art or Science?
The grading system that is used for colored stones today is modeled after the grading system for diamonds. However, diamonds and colored gemstones do not share all the same qualities and that creates problems when attempting to grade a colored stone. When grading a diamond, the stone’s brilliance and fire come into account, while when grading a colored stone these factors are of no importance. For example, in ruby or sapphire, fractures and inclusions sometimes enhance the quality and value of the stone. Therefore, grading systems do not work for each other. When looking at a colored stone, you must look at it as a whole and not just a series of parts as one does when grading a diamond. You still use a similar grading system as diamonds however the fifth C is color coverage. AAA” grade is assigned to a gemstone that is considered to “being near perfect.” Though these stones do show some variation in color, they are clean, well-cut, and polished. AA – Semi clean, well cut and polished gemstones are graded as AA.
Gemstones and the 5th ‘C’ – Color Coverage
A gemstone is a mineral, rock, or petrified material that after being cut or polished is used in jewelry or as a collectible. When looking for a gemstone, as you would a diamond, you look for the same 4c’s, plus one more. You also look for the fifth C, color coverage. However, gemstones do not have a grading system for the 5c’s as diamonds do, therefore you must be an informed consumer.
When judging the quality of a colored gem, the tone is an important consideration. Before buying, it’s always a good idea to consider the lighting conditions under which it will be worn. Also, view gems at arm’s length and look for those that are attractive even at a distance. Exceptional gems will look great under all lighting conditions and viewing distances.
Gemstones tend to form under quite unique circumstances and each stone is comprised of different minerals. Therefore, the minerals leave a distinctive set of marks and inclusions. In the case of a gemstone, inclusions will not necessarily detract from the beauty of the stone. When you are examining the gemstone’s clarity you must look at the gemstone as an individual stone. Some gems have naturally fewer marks or inclusions, while some have more than others. The best values are available in gemstones that are moderately included. Exceptional gemstones with few or no inclusions can be found, but they can command much higher prices.
Cutting standards are far more relaxed for colored gemstones than for diamonds. There are no real geometrical standards for maximizing the brilliance or color of the gem because there are so many different colors of gemstones. Rare gemstones are at times cut for size without regard to their color because it is so hard to find something with that hue.
It is important to examine the symmetry of the gemstone in its setting. A gemstone that is well-cut is symmetrical and reflects light evenly across the surface of the stone, the polish is smooth and there are no nicks or scratches. When a stone is cut asymmetrically it indicates low-quality.
Gemstone Carat Size
Because different gemstones have different densities or specific gravity, two gemstones that appear to be the same size may actually have very different weights. In the jewelry industry, it is common to refer to millimeter size and then convert to carat weight. It is very important to ask about the dimensions of a gemstone to make sure that the majority of the weight of the stone will be seen when set. If putting in the setting you would base it off the millimeters of the stone, not the weight.
Gemstone Color Coverage
There are a few factors that tend to influence color coverage. They include proportions, fluorescence, and inclusions. Suitable cutting is also crucial in maximizing color coverage. When a gem is cut too shallow, all short light paths are apparent, which reduces saturation in many areas. When that occurs, the areas are called windows or zoning. In addition, when the gem is cut too deep, it allows light to exit the sides, which creates dark areas called extinction. When there is an area that permits total internal reflection, it displays the most highly saturated colors which are called “brilliance”.
When shopping for a gemstone, the qualities are similar to a diamond. However, you should still be aware of the differences in order to suit your taste and budget.
Gemstone Durability and Hardness
A common misconception is that durability and hardness are the same things, but they are not. The main quality of gems is that they must be resistant to abrasions. The scale that determines hardness is the Moh’s hardness scale; it lists ten minerals and lists them on a scale from 1 to 10. Each numbered mineral has the potential to scratch the mineral listed below it. A diamond is a 10 on the Moh’s scale and is the hardest known natural substance. The mineral that comes in at a 9 on the Moh’s scale is that of corundum, which is what rubies and sapphires are made from. Topaz is an 8, which makes a topaz easier to break. Quartz, which is what amethyst and citrine are made of, is a 7 on the scale and is the softest translucent material. The scale goes down in number to include the remaining minerals and gemstones.
In terms of colored gemstones, transparency does not have to be an important factor. Sometimes, when a colored stone has inclusions the color is enhanced. Most transparent gemstones are cut into faceted stones.
When a gemstone is pleochromatic, it means that when light is reflected, different colors appear in the stone. Ruby and sapphire have two color shades and are pleochroic; in ruby, for example, yellow-red and purplish-red, which distinguishes it from garnet and red spinel, which has no pleochroism. Iolite displays lavender-blue, gray, and pale yellow when viewed from different angles.
Most gemstones available today have been enhanced or treated in some way. Heat treatment is a common enhancement that has been used for centuries. The jewelry industry recognizes heating and hardened resins as acceptable permanent treatments for most gemstones. Non-heat-treated gemstones are very expensive. Precious stones with non-heat treated can be more than diamonds and are often certified by AGTA.