Maybe the best part of color grading is the simplicity of the tools required. You will not need microscopes, refractometers, polariscopes, etc.
If you scan the list of “elements” of our grading system, you would see that few instruments are required.
Eyes are the most important tool you have. Train them well
Consistency – The intensity of light that reaches an object varies inversely with the square of the object’s distance from the light source. So holding the gemstone the same distance from your light source .. and observing the gemstone at the same distance from your eyes is extremely important.
You need to be able to inspect types of inclusions, surface blemishes and facet alignment
Light source – Get a good light source and learn how to use it. The most important point is using the same light source every time you grade
Incandescent light is not a good grading light regardless of it’s strength. Incandescent bulbs don’t have a high enough “color temperature” to provide a full mix of wavelengths for proper grading of blue stones.
Many fluorescent lamps have similar shortcomings. An ideal color grading light would have a “color temperature” between 6,000 & 7,000 degrees Kelvin and a balanced spectrum.
“Noon sunlight” has a color temperature of about 4,800 to 5,300 degrees Kelvin.
Each color / hue has a unique wavelength. Sunlight, daylight and what we call ‘white light’ are all made up of a mixture of colors / hues (wavelengths) that make up the visible spectrum (red, orange, yellow, green, blue, violet).
Red has the lowest frequency (longest wavelength) and violet is at the opposite end of the spectrum. Some recommended light sources for grading colored gemstones
- A Xenon short-arc lamp (6000 deg. Kelvin) may be best, but they are not widely available
- Another is a 150 watt as produced by Osram GMBH
- Mitsubishi Daylight 99 (5,500 degrees Kelvin), but requires a special and expensive fixture
- Sylvania Design 50 (5,000 deg. Kelvin) – widely available, fits standard fixtures and is affordable.
When no Gem Grading Light is Available
- Never grade in direct sunlight
- Use skylight (light coming indirectly from the sun)
- Use north skylight if possible
- Use that same light source each time
- Grade the same time of day .. mid-day is best
- Don’t grade in overcast or cloudy weather
- Don’t grade near a tinted window