One of the four Precious Gems, the deep-green gemstone Emerald (a variety of the mineral beryl) is the Birthstone for May. It is also the Zodiac stone for the constellation of Cancer, the gemstone for the 20th and 55th Anniversaries, the Day Stone for Friday (and an alternate for Thursday), the Season Stone for Spring, and is mentioned in the Bible.
Emerald is found in many places around the world, but the best deep green transparent crystals come from only a few locations, including Columbia. Emerald is nearly always "oiled" to fill (and thus hide) the minute fractures that otherwise mar a perfect transparency. Modern treatment replaces oils with plastics or other materials that do not evaporate or degrade over time. As long as the treatment does not alter the color, it is considered acceptable.
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