other names: Agate, onyx
mineral class: Oxides and hydroxides
chemical formula: SiO2
Chemical elements: Silicon, oxygen
Similar minerals: /
colour: multicolored (inter alia brown, yellow, red, black)
shine: Glass gloss
crystal structure: trigonal
mass density: 2,8
Mohs hardness: 6,5 - 7,0
stroke color: White
transparency: transparent to opaque
General information about the agate:
Of the agate describes a finely crystalline quartz from the group of oxides and chalcedony, which was named after the old, no longer common name of the southern Italian river Achates (now Dirillo), his alleged first locality. Until the end of the 18th century, the still known name Onyx was in use. The agate consists of the elements silicon and oxygen and is characterized by a varied coloration with a clearly recognizable concentric stripe drawing, which results from the characteristic band structure. Untreated specimens have a glassy gloss and may be slightly transparent or opaque. They appear multicolored in different brown and yellow tones, light blue, soft green, orange, red or black. Stones with a bright blue and pink or an intense green are invariably artificially colored.
Occurrence and localities:
Agates are common throughout the world and are found mainly in former volcanic landscapes near hot springs, but are often found in cavities of ore deposits, in corals, snail shells and petrified wood, as well as in fossil bone finds. Countries that have significant Achat sites include the United States, Mexico, Uruguay, Brazil, Madagascar, Botswana, Zimbabwe, New Zealand, Australia and India. In Europe, Germany, the Czech Republic, Russia, Bulgaria and other countries in the east are the most important sites.
History and usage:
Agate has been highly valued by humans for many millennia. Agates were originally made into hunting instruments and tools long before their optical advantages were recognized and used purposefully. Since ancient Egypt, these stones have been used primarily for the production of precious and colorful pieces of jewelry such as earrings, necklaces and brooches, and as a material for inlays such as jewelry boxes, vases and other vessels. Today agates used for the production of extravagant pieces of jewelry are usually dyed, whereby several methods can be used. Originating from Brazil specimens are particularly suitable for dyeing, since they have few cracks and at the same time a whitish or gray starting color. The agate is first cleaned with surfactants and dried for several days at high temperatures to prepare it for the coloring solution. If a bright blue or green color is desired, the iron oxides in the stone must be removed under the influence of acids. Subsequently, the stone is placed in a sugar solution or sulfuric acid, so that the carbohydrates are decomposed and the resulting carbon allows the coloring. To obtain an attractive appearance, agates are also heated, oiled, varnished or waxed.