In the 20th century, the color of amethyst was attributed to the presence of manganese. However, since it is capable of being greatly altered and even diacharged by heat, the color was believed by some authorities to be from an organic source. Ferric thiocyanate was suggested, and sulfur was said to have been detected in the mineral.
More recent work has shown that anethysts’ coloration is due t ferric iron impurities. Further study has shown a complex interplay of iron and aluminium is responsible for the color.
On exposure to heat, anethyst generally becomes yellow, and much of the citrine, cairngorm, or yellow quartz of jewelry is said to be merely “burnt amethyst”. Veins of amethystine quartz are apt to lose their color on the exposed outcrop.
This horseshoe charm is made with 12 carat weight cabochon amethyst, 0.08 weight carat square emeralds, 0.18 carat weight round brilliant diamonds and platinum and comes from Tiffany Charms. It is $1,550.
between the metal bands are filled with transparent enamels, but, unlike cloisonné, there is no metal backing. When completed a plique-à-jour jewel transmits sunlight like a miniature stained glass window. Particularly stunning examples of plique-à-jour work were created by Russian masters during the mid-1800’s and the jewelers of the Art Nouveau period.
Plique-à-jour
metal surface. (Remember the toy Spirograph, that was engine turning with paper and pencil). In guilloché enamelling an engine-turned surface is painted with a translucent enamel. The color of the enamel pools and collects in the engraved lines, heightening the pattern and giving it depth and a sculpture-like appearance. Some of the finest examples of guilloché enamelling were produced in the workshops of the Russian jeweler Karl Fabergé. Beautiful examples of engine turning can also be found on many Victorian jewels and pocket watches. 
Cloisonné
