Chemistry: Lazulite (Na, Ca) 8(AlSiO4) 6[(SO4, S, Cl) 2], Sodalite, Calcite, Haüyne, the chemistry changes depending on the deposit and the places inside it. Specific gravity: 2,5-3,00, more if a lot of pyrite is present Hardness: 4,5-5 in the Mohs's scale Color : ultramarine blue, greenish blue, violetish blue This stone is not really a mineral like most others gemstones, it is in fact classified as a rock Indeed it is a complex mix of different minerals, mostly Lazulite, which gives its wonderful ultramarine blue, but also Calcite, Sodalite, hauynite and the typical inclusions of pyrite : "the gold shine by spots" say Pline the Old in his Historica Naturalis. That's mostly what differentiates it from its cousin, the Sodalite which can be very close in appearance.
The Lapis has been used since very ancient times, the first traces of it in the jewelry are over nine thousands years old. Occurring in Babylonian mausoleums, Egyptian pyramids and also in the pre-Columbian civilizations, it's a symbol of luck and friendship and was also supposed to guide the dead into the hereafter. It's name is a compound of "lapis", the Latin word for stone, and the Arabic word "azul", denoting its colour, as blue as the sky. Curiously, most of these antique stones were found to come from the world's major deposit: Afghanistan. It was so rare and appreciated that one made thousands and thousands of miles by feet, camels and horses in order to bring it to the final customer, of course at a substantial price. That's the story of all antique caravans. But the Lapis was not only used as a jewel; it was also renowned as a pigment in the noble art of painting, especially in the Renaissance.
The better blue was obtain by mixing resin, olive oil and mastic with the crushed stone, according to the recipe of the XIIth century's monks. Let's talk about the deposits. The most renowned and virtually all the best quality Lapis comes from Afganisthan, in the little valley of Sar-e-Sang, where no car can penetrate due to the very bad roads, the only mean of transportation is the mule. That's why no huge blocks can be found on the market, the biggest are those a mule can carry. The Lapis can only be found in small deposits, mainly primary ones like in Chile ( Flor de Los Andes), Russia ( Baïkal lake, one near the well-named Lazurnïa river), USA ( Colorado and California), Canada and Italy . It occurs also in Myanmar, at Thabanpin near Mogok, where there are two deposits, the first is primary and the second is alluvial (the origin of which remains unknown by non-locals). Common enhancements for this stone are dyeing and coating, with wax or resin in order to improve the luster, and when the stone has been dyed, it stabilizes the colorant which can be constant or not, depending on its chemistry and the porosity of the material.
These treatments can be detected by the hot point method for the coating and in testing the dyeing with acetone on a swab of cotton or checking the stone to look for color concentrations. Some others stones can also be dyed in order to imitate Lapis but none have its specific appearance and these imitations are easily recognizable. Harder to find is the synthetic Lapis, produced by Gilson, it has the same same optical and physical properties, but hopefully not really the same visual aspect.
This synthetic has an overall uniformity of grain size and color, and the pyrite inclusions that can occur don't seem like the natural one. A specialist can easily make the difference with the naked eye. The best quality of Lapis Lazuli is the ultramarine one, uniform in color, with just a few pyrite inclusions which improve its specific blue by contrast. The color of lapis is "deep, bold and solid", no other blue opaque gem comes close........ |