Sapphire Specialist Mini Course
Grading Tools for Sapphire
Purchase Sapphire Specialist Mini CourseBeloved for its celestial blues and violets and exceptional hardness and durability, the sapphire is a long standing favored gemstone. Take an in-depth look at sapphires, from how they form to how to appraise them. Looking to buy or sell a sapphire? Learn how to determine sapphire quality. Every sapphire lover will learn something new in this course.
Before grading, make sure you're dealing with natural, gem-quality corundum — rubies or sapphires — rather than lab-made material or entirely different gemstones. You'll need some basic gemological equipment to measure the physical and optical properties of your gems.
A simple loupe is the best first tool to use when identifying gemstones. Look at the stone's inclusions and growth planes. Are these consistent with natural corundum? A loupe can also help you determine if a gemstone has been faceted well. Although not a definitive test, synthetics and imitations typically don't receive quality cuts.
A microscope can give you a closer look at the gem and its inclusions, helping you identify it.
The next set of properties you'll need to analyze in difficult separations are refractive index, specific gravity, and light absorption spectra. To measure these, you'll need a refractometer, a modified or specialized balance or electronic scale, and a spectroscope. Make sure that all properties of your test stone are consistent with those of natural ruby or sapphire.
These tools, in particular loupes and microscopes, can also help you identify some common corundum treatments. They can even help detect if a ruby or sapphire is unheated — a very rare thing, indeed. However, be aware that some glass-ruby hybrids and glass clarity enhancements are difficult to distinguish with basic equipment.
Grading the Four Cs of Ruby and Sapphire
Once you're sure you have a natural ruby or sapphire, you'll want to grade it using the Four Cs: color, clarity, cut, and carat.
For a ruby or sapphire, color is by far the most important factor in its grade. You may want to use a master set of stones to help you determine the color. Apps like Gemewizard can also help. However, be careful about the color settings on your phone or monitor.
You can only determine color with your own eyes, but it still takes some expertise to accurately judge gemstone color. To better understand color, it's best to separate it into hue, tone, and saturation.
For this, you'll need a workspace with natural north light or adequate artificial lighting. Make sure that you consistently grade your wares under the same lighting conditions.
Hues are the basic colors of red, orange, yellow, green, blue, violet, and purple. Most gemstones have a primary and secondary hue. Some have more than one secondary hue. For ruby, the primary color must be red. If the primary color of a corundum gem is anything else, it's a sapphire.
Next, describe the tone. This is typically on a scale of zero to ten, where zero represents the lightest, colorless gems and ten the darkest. If you decide a ruby's tone is less than five, you may want to call it a pink sapphire instead.
Finally, describe the gem's saturation or "intensity" on a scale of one to six, where the most highly saturated gems receive a six. Few gemstones can reach the highest "vivid" saturation, but exceptional rubies and sapphires can.
Unlike diamonds, colored gemstones don't require magnification for assigning clarity grades. Top-clarity rubies and sapphires will be eye-clean, while lower grades will have small inclusions. The lowest grades will have noticeable inclusions that detract from the stone's beauty.
Still, a loupe and a microscope can aid in clarity grading if you have trouble identifying clarity characteristics with your eyes alone.
Rubies and sapphires often receive native cuts at the mine. Intended to preserve weight, these poor cuts can make the gems unattractive, so it's important to accurately describe the cut quality. Expertly cut rubies and sapphires will show better colors and optical performance.
Again, the best ruby and sapphire grading tools are your eyes and perhaps a loupe. Check for symmetry, windowing, and extinction. Look out for bulging areas and a thick pavilion.
More Ruby and Sapphire Grading Tools
Of course, to measure a gem's carat weight, you only need an accurate scale calibrated to measure small objects like gemstones.
An ultraviolet (UV) light can also be useful, since the chromium content in rubies causes fluorescence. It can also help you to identify glass-filled rubies.
A geologist, environmental engineer and Caltech graduate, Addison’s interest in the mesmerizing and beautiful results of earth’s geological processes began in her elementary school’s environmental club. When she isn’t writing about gems and minerals, Addison spends winters studying ancient climates in Iceland and summers hiking the Colorado Rockies.
When you join the IGS community, you get trusted diamond & gemstone information when you need it.
Get started with the International Gem Society’s free guide to gemstone identification. Join our weekly newsletter & get a free copy of the Gem ID Checklist!