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Obsidian artifacts are common at Tell Brak, one of the earliest Mesopotamian urban centers, dating to the late fifth millennium BC. [44] Obsidian was valued in Stone Age cultures because, like flint , it could be fractured to produce sharp blades or arrowheads in a process called knapping .
Flint – Cryptocrystalline form of the mineral quartz; Geyserite – Form of opaline silica often found around hot springs and geysers; Greywacke – Sandstone with angular grains in a clay-fine matrix; Gritstone – Hard, coarse-grained, siliceous sandstone; Ironstone – Sedimentary rock that contains a substantial proportion of iron ore
Obsidian projectile point.. Obsidian is a naturally formed volcanic glass that was an important part of the material culture of Pre-Columbian Mesoamerica.Obsidian was a highly integrated part of daily and ritual life, and its widespread and varied use may be a significant contributor to Mesoamerica's lack of metallurgy.
A mineraloid is a naturally occurring substance that resembles a mineral, but does not demonstrate the crystallinity of a mineral. Mineraloid substances possess chemical compositions that vary beyond the generally accepted ranges for specific minerals, for example, obsidian is an amorphous glass and not a true crystal; lignite is derived from the decay of wood under extreme pressure ...
Opals too thin to produce a "natural" opal may be combined with other materials to form "composite" gems. An opal doublet consists of a relatively thin layer of precious opal, backed by a layer of dark-colored material, most commonly ironstone, dark or black common opal (potch), onyx, or obsidian. The darker backing emphasizes the play of color ...
Aphanitic (a = not, phaner = visible) rocks, in contrast to phaneritic rocks, typically form from lava which crystallize rapidly on or near Earth's surface. When extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are ...
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A chiral tetragonal form called α-cristobalite (space group either P4 1 2 1 2, No. 92, [9] or P4 3 2 1 2, No. 96, at random) occurs on cooling below about 250 °C at ambient pressure and is related to the cubic form by static tilting of the silica tetrahedra in the framework.