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Geosciences LibreTexts

7.10: Color

  • Page ID
    3498
  • Color is one of the most important (if not the most) factors that determine the beauty of gemstones.
    While most other aspects in gemology are abstract and well defined, color is not. It is very subjective and the sensation of color differs from person to person as much as the visions of opponents in politics, religion or humor. To some, even the absence of color is preferred and the beauty is perceived from the optical properties a gem might posses (as the brilliance in diamond or reflection from rutile needles in Kashmir sapphire).

    Note

    Believe it or not, but the above white space is made up from 3 different colors (red, blue and green). Hold your loupe close to the screen and observe the powers of light.

    Basic

    "a phenomenon of light (as red, brown, pink, or gray) or visual perception that enables one to differentiate otherwise identical objects"

    Merriam-Webster Online Dictionary

    As can be read from the above definition of color, to humans color is a perception constant. Merriam-Webster defines perception as "awareness of the elements of environment through physical sensation".

    From this can be concluded that the perception of color is a sensation that is different from person to person and is influenced by circumstances. People who are tired will most likely be less sensitive to color than those who are not. Photographers are well aware that objects have different colors in different lighting conditions. Furniture makers know that one should use only the fabric from the same dyed roll when covering a couch, as do tailors when making a suit as the circumstances in dying the fabric may change rapidly, but subtle, when the dye-bath is cleaned or needs a new mix (causing a phenomenon named metamerism).

    The same applies to gemstones. Gems look different under changing lighting conditions and a gemologist should be very aware of this. Blue sapphires look best during afternoon hours, while rubies are at their best behavior before noon and in the evenings. Some gemstones even dramatically change their hue between daylight and incandescent light.

    Causes of color

    main article: Causes of color

    What we see as color is merely the reflection of light from objects.
    As white light (from the sun or other source) shines on a gemstone, some of the wavelengths will be absorbed by the gem while others are reflected from and/or transmitted through the stone. It is the combination of these left over ("residual") wavelengths that are collected at the back of our eyes (the retina) and interpreted by our brains, that defines the color of an object.
    How exactly an object absorbs light is explained elsewhere.

    In order for color to exist there must be 3 conditions met:

    1. color vision
    2. an object
    3. light shining on an object

    From these 3 requirements, one asks a few philosophical questions, as: does an object have color in the dark? The answer should, of course, be no. In fact, gemstones do not have color in light either, the color is produced by the interpretation of our eyes and brain. Which is different from one person to the other.
    Under different lighting conditions, gemstones appear to have a different color, which can be easily demonstrated by shining different colors on a white object, like a wall. If you shine "white" light on the wall, none of the spectral colors are absorbed by the wall and all is reflected back to your eyes. Making the wall look white. When one shines a green light on the wall, only the green portion of the spectrum can be reflected, hence the wall appears green.

    Mixing colors

    From a young age most of us are taught that if we mix red and green together, we get a black color (or at best a dark gray). The reason for this is that we were mixing paint. Mixing paint is subtractive color mixing.
    For light, it works differently. When we would mix red light with green light, the result would be yellow light and this is termed additive color mixing.
    Similarly if we mix red, green and blue paint together, the result will be black. But if we mix red, green and blue light together we get white light.

    So when we think of light, we should completely forget about our Picasso minds.

    File:Additive color mixing.jpg File:Subtractive color mixing.jpg

    Figure \(\PageIndex{1}\): Additive color mixing of light

    Figure\(\PageIndex{2}\): Subtractive color mixing of paint

    With additive color mixing, we start with no light and add colored light. When we are in a dark room and we shine a red light (as from a torch) on a wall, that spot will appear red. Now when we add light from a green torch to it, we get a yellow spot on the wall. If we then at blue light to it, the spot appears white.

    For subtractive color mixing, it works reversed. We shine white light on an object and the object absorbs portions of the light, subtracting that particular color from the light.

    Lighting conditions

    Proper lighting is crucial when attempting to color-grade gemstones. Selecting the correct lamps may greatly affect the color you perceive.

    Daylight

    Sunlight has color temperatures between 2,000 and 28,000° Kelvin. In the mornings the color temperatures are lower and look more red to yellow. Around noon the color temperature will be around 5.500° Kelvin and in the afternoon the color appears to be more blue with a high color temperature. In the evenings, the color temperature drops and the light will appear again more yellow to red. As one might suspect, different times of day will have an influence on the colors of gems.
    The standard in grading colored stones is light at 5,500° Kelvin, that is when the light from the sun is mostly white.

    Fluorescent lamps are produced to mimic this daylight from the sun and a wide range of good quality lamps are available. Pay good attention to the color temperature when buying one.

    Incandescent light

    Incandescent light is light from a hot object like a flame or a tungsten lightbulb. Usually, these lamps cannot be produced with a color temperature over 3,000° Kelvin and are therefore not suitable for the color-grading of gemstones.