Quartz (XXIV)Regular price $499.00
Mkobola, Nkangala District
Mpumalanga Province, South Africa
4.7 x 2.3 x 1.7 Inches
12 x 6 x 4.5 Centimeters
Quartz has been known and appreciated since pre-historic times. The most ancient name known is recorded by Theophrastus in about 300-325 BCE, κρύσταλλος or kristallos. The varietal names, rock crystal and bergcrystal, preserve the ancient usage. Quartz is one of the most common minerals found in the Earth's crust.
If pure, quartz forms colorless, transparent and very hard crystals with a glass-like luster. A significant component of many igneous, metamorphic and sedimentary rocks, this natural form of silicon dioxide is found in an impressive range of varieties and colours.
Quartz is found as individual crystals and as crystal aggregates. Well crystallized quartz crystals are typically six-sided prisms with steep pyramidal terminations. They can be stubby ("short prismatic") or elongated and even needle-like. In most environments quartz crystals are attached to the host rock and only have one tip, but double-terminated crystals are also found.
Quartz is best known for the beautiful crystals it forms in all sorts of cavities and fissures. The greatest variety of shapes and colors of quartz crystals comes from hydrothermal ore veins and deposits, reflecting large differences in growth conditions in these environments (chemistry, temperature, pressure). Splendid, large crystals grow from ascending hot brines in large fissures, from residual silica-rich fluids in cavities in pegmatites and from locally mobilized silica in Alpine-type fissures. An economically important source of amethyst for the lapidary industry are cavities of volcanic rocks. Small, but well-formed quartz crystals are found in septarian nodules, and in dissolution pockets in limestones.
Amethyst: Typically a purple variety due to iron impurities and irradiation - Iron is captured during crystallization and surrounding materials release gamma rays irradiating the iron. Occurs worldwide with documented uses from Ancient Greek and Egyptian cultures.
Cactus Quartz: Large amethyst and/or citrine quartz crystals covered with smaller spiky crystals - Only found in Boekenhouthoek, South Africa
Vera Cruz: Elongated and prismatic crystals only found in Vera Cruz, Mexico
“Grape Agate”: A popular but misleading name - the “agate” is actually botryoidal amethyst quartz.
Smoky Quartz: Smoky brown and grey hues created by irradiation of trace aluminum captured during crystallization. Varies from translucent to opaque depending on amount of surrounding radioactive emissions. Occurs around the world with popular localities including Colorado, Scotland, Africa and Switzerland.
Herkimer Diamond: Local name for clear, doubly-terminated, usually quite clear and highly lustrous, quartz crystals from cavities in dolostones in Herkimer Co., New York, and surrounding areas, USA. They are frequently doubly terminated with prism faces about as prominent as the pyramidal terminations. They were originally (1790) named "Little Falls Diamond(s)". There are similar terms for quartz crystals from numerous other worldwide locales, all formed under low-temperature hydrothermal conditions in sedimentary rocks and often containing hydrocarbon inclusions.
Herkimer diamonds have been popular with collectors for more than 100 years and various localities for them in Herkimer Co. are still producing many specimens each year even though it requires a lot of hard manual labor to find good specimens. Credit Mindat.org
Faden: Forms in fissures in the host rock that widen slowly and steadily. Quartz crystals inside the host rock will rupture when the fissure opens. In a silica rich solution, this rupture will heal quickly, forming a crystal that is attached to the opposing rock walls and bridges the new opening. While the fissure continues to open steadily, the crystal will also continue to crack and heal. Because growth is much quicker on fractured surfaces than on regular faces and because it leads to small regular faces on the opposite conchoidal fracture surfaces that do not perfectly match, some of the growth solution is included in the crystal. The repeated rupturing and healing leaves a scar of liquid and gas inclusions in the crystal: a white thread, the "faden". Credit Mindat.org
Prase: Originally, the varietal name "prase" was applied to a dull leek-green colored quartzite (a rock, not a mineral*); but over the years it has also been applied to other materials, particularly a green colored jasper of similar color. For perhaps more than a century it was restricted to granular micro-crystalline varieties of quartz and the original quartzite; but in recent years euhedral crystals of quartz having a similar leek-green color have had the term applied to them as well, expanding the definition beyond micro-crystalline forms. Now it is simply a color descriptor for quartz: If it is leek-green, it is called "prase" - whether it is micro- or macro-crystalline, and no matter what causes the color. Basically, the term no longer has any scientific rigor - it has become a general term; it can't even truly be called a varietal name any longer - since it covers more than one material. Credit Mindat.org
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Thumbnail: 0.2 to 3 Centimeters / .0625 - 1.118 Inches
Miniature: 3+ to 5 Centimeters / 1.125 - 1.96 Inches
Cabinet: 5+ to 9.5 Centimeters / 2.0 - 3.7 Inches
Large Cabinet: Greater than 10 Centimeters / Greater than 4 Inches
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