Fluorescence
If you put diamonds under an ultraviolet lamp, some of them will glow blue.
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This effect is called fluorescence or luminescence. It does not interfere with the shine and play of the diamond in any way in normal light and is completely invisible without ultraviolet light.
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Fluorescence can be of varying intensity. Some diamonds do not have it, such stones will be transparent or dark in ultraviolet light. This is the category of investment diamonds, they are more expensive, as they are rare in nature.
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Medium and Faint Slight fluorescence is average. These stones have a slight blue glow. These are also rare diamonds.
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Strong and Very Strong fluorescence has a bright strong glow. These stones are a little cheaper. They are common and may have a loss of shine. Then the stone looks cloudy.
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The standard glow color is blue, but stones with yellow, orange, red, green, white glow colors are considered undesirable and are sold much cheaper.