Description
Obsidian is a naturally occurring volcanic glass. The name derives from a Roman named Obsius, who is said to have discovered a stone resembling obsidian in Ethiopia in ancient times. Obsidian is formed by the rapid cooling of lava with a water content of up to 3-4%. If the volatile constituents (water, carbon dioxide) content is higher, pumice is formed because the released gases act as a blowing agent. The formation of volcanic glass is highly dependent on the viscosity of the molten lava, which in turn is strongly influenced by the silica content. The higher the silica content, the slower the fluid flows. Most obsidians have a silica content of 70% or more and are therefore classified as rhyolites. Less frequently, trachitic, andesitic, and phonolithic obsidians are also found, which have a lower silica content. The color of obsidian varies considerably, depending on the impurities it contains. Its color is usually dark green to deep black, although granite with a similar composition is a light-colored material. This is usually caused by very finely divided hematite or magnetite. Snowflake obsidian contains inclusions of spherically grown structures, usually of feldspar or cristobalite, called spherulites. They form through spherical crystal growth from a single central growth nucleus. Obsidian is a metastable material that, over time, reverts to its crystalline, more stable form. Volcanic glass is therefore unknown from the Paleozoic and Precambrian eras. In the Stone Age, obsidian was a coveted raw material because it forms razor-sharp cutting edges. It was used for knives and weapons. Today, it is a popular material for art objects and is also used as a semiprecious stone in jewelry. It is then cut as if it were a crystal.
Quartz is a form of silicon dioxide, SiO2, and is one of the most common minerals in the Earth's crust. It represents more than 12% of the Earth's crust by volume (found in granite and sand, among other places). Quartz has two modifications: α-quartz is trigonal, and β-quartz (a higher-temperature form) is hexagonal. Silicon dioxide also occurs in many other structures, such as α- and β-cristobalite, keatite, tridymite, coesite, stishovite, melangophlogite, and lechatelierite. Although quartz consists of silicon dioxide, it is often classified as a tectosilicate. This is because quartz forms a network of SiO4 tetrahedra, with each oxygen atom shared by two silicon atoms. Individual SiO2 molecules only occur in gaseous silicon dioxide. Quartz can be very pure, as in rock crystal, but often contains impurities. These impurities can consist of ions that are incorporated into the crystal lattice during crystal growth. Quartz crystals can contain between 13 and 15,000 ppm Al3+, between 9 and 1,400 ppm Na+, between 3 and 300 ppm K+, and traces of Fe3+, Ti4+, P5+, H+, and Li+. Aluminum and iron, in combination with ionizing radiation, are responsible for the color of some quartz varieties. Furthermore, quartz crystals can grow over other minerals, as is the case with rutile quartz. Quartz can form both large crystals and microscopic aggregates. The largest quartz crystals are found at Verloren Farm in Namibia. Quartz crystals up to 20 meters, and possibly 50 meters, and dolomite crystals up to 2 meters are found there. When quartz is melted, it often forms a glass upon rapid cooling, as the crystallization process takes a considerable time. Quartz is a piezoelectric material and the vibrations that can be induced in the crystal lattice are used in electronic equipment such as quartz clocks and radios.
