Silica Group of Silicate Minerals
Quartz was the mineral upon which the Stone Ages were based. With few exceptions, most early stone tools were fashioned of quartz. Outcrops of quartz that were suitable for tool manufacturing were targeted by some of the earliest known mining activities and the mined quartz was traded across vast distances, even before humans began to establish agricultural societies.
Even in our modern world, quartz is one of the most widely used minerals, though few people are aware of its many contributions. One of its most common uses is also its most transparent, as quartz is the source of most of our society’s glass- from windowpanes and crystal goblets to eyeglasses and cathedrals’ stained glass windows. Quartz is one of the most common minerals because it is chemically and physically stable at Earth’s surface conditions. It is a significant component of hydrothermal veins and felsic igneous rocks, and is often the dominant mineral in sandstones and siltstones, as well as their metamorphosed equivalents.
Quartz occurs in a wide range of varieties under a bewildering array of informal names, such as jasper, flint, tiger’s eye, amethyst, citrine, chalcedony, onyx, opal and agate. Because of their abundance, the quartz varieties are often subdivided into informal groups. One of the most common subdivisions is ‘chert’, a term collectively used for all the quartz varieties that have crystals too small to be seen without a microscope. In this use, ‘chert’ is simply a subset of ‘quartz’.
Description and Identifying Characteristics
Despite its variable appearance, Quartz’ hardness (greater than that of glass or most metals) and lack of cleavage make it relatively easy to distinguish from other minerals. Ancient Greeks called quartz ‘krystallos’, which means ‘ice’, a reflection of the clarity of quartz’ clear transparent crystals. Although most quartz crystals are clear and colorless, slight chemical impurities create a number of color varieties, some of which are common enough to have their own names. Translucent purple crystals are called amethyst which has been designated as February’s birthstone. Translucent pink to reddish quartz is known as rose quartz, while translucent gray and cloudy white crystals are respectively called smoky quartz and milky quartz. In a similar manner, quartz that is composed of microscopic crystals (chert) can occur in a wide variety of colors. The most common chert color varieties are flint (dark gray to black), jasper (red to red-brown), and onyx (mixed white and black). Agate is another variety of chert that exhibits distinct bands of color that formed as the chemistry of the fluids moving through its host rock changed slightly.
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