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Lapis lazuli

earth science Maturity 7-9 Next article →

Lapis lazuli is a deep-blue metamorphic rock. It is highly valued as a semi-precious stone because of its intense color. The name comes from the Persian word *lāžward*. This word is the root for many terms meaning blue, such as the English word "azure."

Lapis-Lazuli microscope x240
Lapis-Lazuli microscope x240

This rock is a complex mixture of several different minerals. The most important component is lazurite, which usually makes up 30% to 40% of the stone. Lazurite is a blue feldspathoid silicate mineral from the sodalite family. Lapis lazuli also contains calcite, which appears as white streaks. You will often see pyrite, which looks like metallic yellow specks. Some samples also contain minerals like diopside, enstatite, or mica. The rock typically forms in crystalline marble through a process called contact metamorphism.

The famous blue color is caused by a specific chemical mechanism. Inside the crystal structure, there are tiny parts called radical anions. Specifically, the trisulfur radical anion is responsible for the bright blue hue. These anions substitute for chloride anions within the sodalite structure. The presence of disulfur or tetrasulfur radicals can change the color. Disulfur can shift the stone toward yellow, while tetrasulfur can shift it toward red. The blue color occurs because these radicals create a visible absorption band between 595 and 620 nm.

Humans have mined lapis lazuli for thousands of years. As early as the 7th millennium BC, it was being mined in the Sar-i Sang mines in Badakhshan, Afghanistan. These mines have been in use for more than 6,000 years. Lapis lazuli was very important to the Indus Valley Civilisation from 3300 to 1900 BC. Artifacts made of the stone have been found at Bhirrana, the oldest site of that civilization. In ancient Egypt, it was used for amulets and ornaments. It was even used in the funeral mask of Tutankhamun, who ruled from 1341 to 1323 BC.

The history of the stone is also tied to art and religion. In the Middle Ages, Europeans imported the stone to grind into a powder. This powder was used to create a pigment called ultramarine. Many Renaissance and Baroque artists used ultramarine in their work. For example, Masaccio, Titian, and Vermeer all used it. It was often reserved for the clothing of central figures, such as the Virgin Mary.

1665 Girl with a Pearl Earring
1665 Girl with a Pearl Earring

Interestingly, ultramarine has been found in the dental tartar of medieval nuns and scribes. This suggests they may have licked their brushes while working on manuscripts.

Lapis lazuli is found in several major locations today. Afghanistan remains a primary source for the world. Significant amounts are also produced west of Lake Baikal in Russia. In the Andes mountains of Chile, the Inca people once used the stone for jewelry. Smaller amounts are mined in countries like Italy, Canada, and the United States. In California and Colorado, you can find smaller deposits. The stone is used commercially to make jewelry, carvings, mosaics, and vases.

Because the stone is expensive, people sometimes use substitutes. Spinel, sodalite, or even dyed jasper can be used to mimic its appearance. In the 19th century, a synthetic version of ultramarine became available. This changed how artists used the pigment. Today, the stone is still a subject of study and trade. However, mining in some areas, like the Kuran wa Munjan district, involves unethical working conditions. In 2014, armed groups in Afghanistan reportedly made millions of dollars from these mines.

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File:Lapis-Lazuli microscope x240.jpg
Lapis-Lazuli microscope x240.jpg
File:1665 Girl with a Pearl Earring.jpg
1665 Girl with a Pearl Earring.jpg
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