Specific gravity is a measurement of density. As a point of reference, gemologists compare the density of a gemstone to the density of the same mass of water.
Just as it sounds, hardness describes how well a gemstone resists scratches and abrasions. Experts test this by trying to scratch one gem with another. They rank them from 1-10 on the Mohs Hardness scale, invented by German geologist, Friedrich Mohs, in 1812. For example, diamond ranks at 10 on the Mohs scale, the hardest naturally occurring substance. Corundum, which is what makes up rubies and sapphires, is the next hardest on the Mohs scale.
Being aware of a gemstone's hardness can help you determine what type of jewelry they are best suited for, as this will determine how much wear and tear they will be exposed to and can withstand. Hard gemstones can withstand daily wear beautifully and are ideal for everyday jewelry use. This is one primary reason why diamonds are the most widely used gemstone in bridal jewelry. Softer stones are best suited for earrings and necklaces or special occasion rings and bracelets rather than everyday staples. It is important to protect delicate gemstones that are prone to damage.
Rarity presents a gemstone with uniqueness and unparalleled value. These qualities are what increase people’s desire to possess them. Rarity is one of the most decisive factors used to determine the price of gemstones and their elusiveness. Some prevalent stones can also be deemed rare because of exceptional color or clarity, making them more coveted than their typical counterparts.
Everyone agrees that the color of a gemstone is an important characteristic. The colors we see come from the minerals within the stone and how a gem reflects light. For example, the natural, clear color of a diamond comes from its stunning purity and crystalline shape. The presence of other minerals in a diamond can give it unique color hues, such as yellow, pink, or black. Color is the main appeal of vibrant gems like rubies, sapphires, emeralds, and amethysts. Still, many jewelers regard color as the most critical evaluation criterion of all gemstones—hue, tone, and saturation influence the final color of these dazzling jewels.
- Hue: Hue represents the purity of color. Gemstones that display pure colors or only a “slight” hue of other colors in addition to their primary shade are recognized as the highest quality. For instance, some rubies contain slightly orangish-red and slightly purplish-red hues.
- Tone: Tone refers to the depth of a gemstone’s color. It describes how light or dark a gemstone’s color is by using terms such as “light,” “medium-light,” “medium,” “medium-dark,” and “dark.”
- Saturation: This describes how pure or intense a color appears and indicates the degree to which the gemstone is free from brown or gray hues. Gems that are described as emitting “vivid” or “strong” color saturation are free from brown or gray hues.
Much of how we perceive a gemstone’s beauty lies in how intricately light reflects and refracts throughout the stone.
- Reflection - Reflection is the amount of light that bounces off the surface of a gemstone and is returned to the eye. This is what we refer to as “brilliance” or sparkle.
- Refraction & Dispersion - Refraction is how the gemstone bends light in different directions, subsequently separating white light into its spectral colors. This separation is called dispersion and is considered to be the “fire” of the gemstone. Stones that are transparent and translucent have their own reflective index or, in other words, a numerical indication of their dispersion. The higher the R.I. of a gemstone, the more dramatic and substantial the dispersion.
Many gemstones possess unique properties that allow them to display special light effects that make them even more appealing. Most of these optical occurrences are dependent on how the gemstones are cut. Let's explore some of the spectacular special effects that stones are capable of displaying.
- Chatoyancy - When cut in a cabochon, certain gems will exhibit chatoyance: a brightly reflected single line that moves across the surface of the gemstone when rotated. Derived from the French word “chatoyer,” chatoyance means “to gleam like a cat’s eye.” Tiger’s eye is one of the most well-known examples of a gem that displays chatoyance.
- Asterisms - In gemstones with rutile inclusions, light that travels perpendicularly to the three crystal faces will produce a gorgeous star effect on the top of the cabochon. Star sapphires and star rubies are stunning examples of gems that contain asterism. The iconic Star of India is a 563k star sapphire, best known as the largest star sapphire in the world.
- Adularescence - This is the "inner glow" of a cabochon gemstone. When a stone seems to emit light, this phenomenon is called adularescence. An excellent example is Moonstone. When the gem is very clear it exhibits an ethereal blue adularescence. This sheen travels across the surface of the crystal when the stone is turned and observed.
- Iridescence - Iridescence, also known as schiller, is the phenomenon where a gemstone displays different colors dependent on which angle they are viewed. Some good examples of this are the beautiful opal, labradorite, and ammolite gemstones. Opals are formed by the effect of warm water on volcanic glass when water particles become trapped during the cooling process. The iridescence emitted are liquid inclusions. On the other hand, labradorite has an almost metallic sheen because of the variations in its mineral composition.
Some gemstones will display dramatic changes in color when viewed in different lights like incandescent lighting or daylight. Alexandrite is the most well-known example of a color-changing gem when in incandescent light. It will absorb all spectral colors except for red, which is then reflected back to the eye. When viewed in natural light, alexandrite will absorb red light and reflect blue. While it is pretty rare, some sapphires can change color and can shift from pink to green or from blue to purplish-blue, depending on the light it is being viewed in.