Which gem has an unusual dark brown or black color when it occurs as crystals with an Al impurity in pegmatites rich in silica

What is Smoky Quartz?

Smoky quartz is a color-variety of crystalline quartz. It ranges from a light yellowish brown to a brown that is so dark that it appears to be black. Less-desirable specimens have a grayish brown color. When cut as a gem, stones with an orangish brown to a reddish brown color are preferred by many people.

Smoky quartz is a relatively inexpensive gem material because it is abundant, found in many locations, and its brown color is not currently in high demand. It is often found in large crystals of excellent transparency with few inclusions.

The color of smoky quartz is produced when natural radiation, emitted from the surrounding rock, activates color centers around aluminum impurities within the crystalline quartz. Amethyst forms in a similar way, from the activation of color centers around iron impurities.

Explanation:

Treatments

Some specimens of rock crystal can be given the color of smoky quartz by irradiation in a laboratory. The exposure can be controlled to vary the color of the product. This is not often done in a lab because natural smoky quartz is inexpensive and abundant relative to demand. For the same reason, synthetic smoky quartz is seldom produced. Natural smoky quartz with a very dark color is sometimes heated to lighten its color.

faceted smoky quartz

Faceted smoky quartz: Rich brown faceted stones of smoky quartz, all cut from material mined in Brazil. From left to right they are a 5.28 carat pear concave cut measuring 14.84 x 11.01 millimeters; a 7.4 carat round measuring 14.09 millimeters; and a 7.16 carat cushion checkerboard cut measuring 12.19 x 12.15 millimeters.

Uses of Smoky Quartz

Smoky quartz is faceted or cut into beads and cabochons. These gems are often used in rings, pendants, necklaces, earrings, and brooches for people who like the yellowish brown to brown color.

Smoky quartz with a dark brown color is often used in men's rings and cufflinks. During the Victorian Period, dark brown stones from the Mourne Mountains of Ireland were often used in mourning jewelry.

Because smoky quartz has a low price and large pieces are easily obtained, it is a favorite practice material of people who are learning to facet. It is also frequently used as a carving material.

pie chart because it is in percents ( % )