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Clarity grading in coloured gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20/20 vision). [29] The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-clean.
Gemstones of the World revised 5th edition, 2013 by Walter Schumann ISBN 978-1454909538 Smithsonian Handbook: Gemstones by Cally Hall, 2nd ed. 2002 ISBN 978-0789489852 hide
In 1986, California named benitoite as its state gemstone, a form of the mineral barium titanium silicate that is unique to the Golden State and only found in gem quality in San Benito County. [ 80 ] ^ Colorado is the only state whose geological symbols reflect the national flag's colors: red (rhodochrosite), white (yule marble), and blue ...
American Golden Topaz, the largest cut yellow topaz, weighing nearly 23,000 carats (4.6 kg). Chalmers Topaz , a 5,899.5-carat (1.17990 kg) cut topaz. Tsavorite
The classification of the cardinal gems dates back to antiquity, and was largely determined by ceremonial or religious use as well as rarity. [1] The term has largely fallen out of use. The five traditional cardinal gems are: amethyst (purple) – Rare and precious in the Old World until large deposits were found in Brazil. diamond (transparent)
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