Though gems have amazed and delighted man from the start of memory, the science of gemology has been finely tuned in recent centuries. A specialized division of mineralogy, gemology is the science of natural and synthetic gems and gemstones. Gemologists use methodical testing and observation of a gem's physical properties in order to identify and evaluate specimens.
When buying gemstones, it is useful to know how key properties affect it's life, wear, and care requirements. Understanding a stone's attributes will assist you in making an informed choice as to whether or not it is the best gem for you and your intended use.
Based off of the system created by mineralogist Friedrich Mohs, the Mohs Scale of Hardness gives a tangible way to rank the respective hardness of minerals and gemstones. It assigns a number between 1 and 10 to represent the material's resistance or ability to scratch, though the interval values are not equal to one another. A material can scratch the surface of those of the same or lower ranking, but is unable to affect materials of higher ranking. Diamond is the hardest material known to man, topping off the scale at a 10.
Mohs Hardness Rankings:
- 1-2 can be scratched by a fingernail
- 2 can be scratched by copper
- 4-5 can be scratched by steel
- 6-8 can scratch glass
- 9-10 can cut glass
Refractive Index (RI) :
As light shines into a gem, the material forces the beam to slow it's rate of travel. A gem's refractive index signifies the ratio of that speed against light's speed in air. Refractive index differs between gemstones of separate chemical compositions, providing a consistent property for testing use and identification.
Specific Gravity (SG) :
The densities of gem materials differ from one another. A gem's specific gravity signifies the ratio of of it's weight in comparison to that of an equal volume of water at 4 degrees centigrade. A higher ratio indicates a more dense and heavy matter. Using specific gravity, you differentiate the carat weights of gems that are the same size. Example – A 2.0 carat alexandrite (higher SG) would be the same size as a 1.4ct emerald (lower SG).
Chemical Composition :
Each gem material's chemical composition is structured by individual types and quantities of atoms. Chemical formulas create a written outline that identify these specifics arrangements. Example – Chrysoberyl's chemical composition can be expressed as BeAl2O4 (Be = beryllium, Al = aluminum, 0 = oxygen), where there is one beryllium atom for every two aluminum and four oxygen atoms.
In addition, these physical properties of gemstones can be observed in order to help with identification:
- Color – The hue, tone, and saturation of a stone, created by selective absorption of light
- Transparency – How well a material transmits light
- Luster – The quality and intensity of a material's surface in reflected light
- Cleavage – A break in a gem that parallels it's atomic weaknesses
- Dispersion/Fire – The visual effect when white light is separated into colors within a stone
- Heft – How heavy a gem is in relativity to it's size
- Fracture – A break in a gem not attributed to cleavage or parting
- Fluorescence – The visual glow emitted by a material when exposed to ultraviolet (UV) radiation
- Pleochroism – When a gem displays varying body colors when viewed from different directions (dichroism, trichroism)
- Phenomenon – Unique optical effects seen in gemstone
The information on our website has been fact checked by Jessica Neiwert G.G. of Seattle Washington. Thanks for your hard work and we appreciate your participation in the creation of the website and the app.