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Color grading of diamonds was performed as a step of sorting rough diamonds for sale by the London Diamond Syndicate. As the diamond trade developed, early diamond grades were introduced. Without any co-operative development, these early grading systems lacked standard nomenclature and consistency. Some early grading scales were; I, II, III; A, AA, AAA; A, B, C. Numerous terms
Heat can either improve or spoil gemstone color or clarity. The heating process has been well known to gem miners and cutters for centuries, and in many stone types heating is a common practice. Most citrine is made by heating amethyst, and partial heating with a strong gradient results in "ametrine" – a stone partly amethyst and partly citrine.
Peridot is one of the few gemstones that occur in only one color: an olive-green. The intensity and tint of the green, however, depends on the percentage of iron in the crystal structure, so the color of individual peridot gems can vary from yellow, to olive, to brownish-green. In rare cases, peridot may have a medium-dark toned, pure green ...
Amethyst frequently shows color zoning, with the most intense color typically found at the crystal terminations. One of gem cutters' tasks is to make a finished product with even color. Sometimes, only a thin layer of a natural, uncut amethyst is violet colored, or the color is very uneven.
Theodore Maiman says that "because of the low chromium level in these crystals they display a lighter red color than gemstone ruby and are referred to as pink ruby." [8] After absorbing short-wavelength light, there is a short interval of time when the crystal lattice of ruby is in an excited state before fluorescence occurs.
Virtually all gemstones that show the "alexandrite effect" (color change or 'metamerism') show similar absorption/transmission features in the visible spectrum. This is an absorption band in the yellow (~590 nm), along with valleys of transmission in the blue-green and red.