Fluorite-Opal-Jasper (»Tiffany Stone«)

finely dispersed fluorite gives this Fluorite-Opal-Jasper (»Tiffany Stone«) it's purple color

Foto: K. Sieber, www.makrogalerie.de

At the beginning of the millenium many tumbled stones we sold under the trade names »Tiffany Stone« and "Fluorite Opal".

This beige rock with violet veins, which is known in the USA as "Tiffany Stone" is provided by a large number of small deposits in southwestern Utha (USA), which came to be known as "Spore Mountain Lavender Mines". In Europe tumbled stones were sold under the name "Fluorite Opal".

The rock owes its formation to the activity of volcanoes, which deposited in several eruptions large quantities of silica-rich ash layers in an area dominated by limestones. Between the sporadic eruptions quartz-rich sediments were gradually solidified and transformed. Fluorine-containing gases led to an impregnation of the sediments with fluorite. In an older period mainly quartz, quartzite, jasper, agate and flint were formed along the contact to the underlying limestone. Manganese ores associated with the limestone, were incorporated and form brown to black, often metallically shiny veins in the rock.

Lavendeljade (Jadeit - Metagranitoid), Türkei

»Lavender-Jade« (Jadeite - Metagranitoide), Turkey

Foto: K. Sieber, www.makrogalerie.de

The world-famous deposit for this lilac-colored rock is located near Harmancik, about 90 km south of Bursa in Turkey. The deposit covers an approx. 400m thick jadeite-bearing metamorphous rock embedded between mica schists and marbles. It was formed about 80 million years ago from an originally granitic rock (granitoid) by high-pressure metamorphism. The resulting metagranitoid rock consists of quartz, chlororitoid, lawsonite, glaucophane and phengite as well as up to 50% jadeite.

Jadeite occurs in this rock not only in the violet color of »Lavender Jade«. To a small extent you can find it there also in grey, white and green. The violet color variety developed during the metamorphosis process by the circulation of manganese-containing aqueous solutions.

 

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»Llanite Rhyolite«, Madagaskar

»Llanite Rhyolite«, Madagaskar

Foto: K. Sieber, www.makrogalerie.de

A colorful rock with roundish to oval domains of shimmering bluish colors, is marketed as »Llanite«, »Llianit« or »Llanoithe« or simply as "Porphyrite" and "Volcanite". The latter term gives an idea about its formation ("it is from volcanic origin") but it does not specify what kind of rock it is - there are many different volcanic rocks out there. The term "Porphyrite" is an outdated name for a class of volcanic rocks, called "rhyolite" in modern time. The term »Llanite« is based on the place of discovery in Llano County, Texas, USA. Known occurrences of rhyolithic rocks with this specific look are Brazil, Madagascar and the USA.

The samples we have examined in the EPI-Lab, were reported to come from Madagascar. We identified a slightly metamorphosed (alkalifeldspar-) rhyolite with exceptionally large crystals, embedded in a fine grained groundmass. Orthoclase feldspar crystals up to 10mm / 0.4in in size and large quartz and plagioclase feldspar crystals up to 5mm / 0.2in are visible to the naked eye. The analyses of various thin sections revealed a well crystallized groundmass. It consists of quartz, mica and feldspar very rich in inclusions, with no glassy component.

Mariposite-Dolomite

Mariposite-Dolomite

Foto: K. Sieber, www.makrogalerie.de

»Mariposite dolomite« is a white stone with green striationss. It occurs in different places in the Sierra Nevada in California (USA), for example around Coulterville in Mariposa County. Benjamin Silliman Jr. named the stone after this region in 1868. Its occurrence is linked to a contact zone of argillaceous schists and ultrabasic rocks.  Therefore dolomite and quartz coexist together with green phyllosilicates. These silicates are in a large part mariposite (a chrome-muscovite mica) but sometimes also of nickeliferous chlorite.

Minerals of the mica group are classified in white micas and dark micas depending on their appearance. Muscovite is a common member of the white mica group. The pure mineral is colorless and transparent. Variations in its chemical composition modify its physical properties (e.g. flexibility, color). Such varieties bear their own names like »Fuchsite« for a chrome mica or »Phengite« for SiO2 rich mica and »Mariposite« which is rich in chrome and SiO2. Sometimes Mariposite is also called »Chromium phengite« because of it's high content of chrome and SiO2.

Maw-Sit-Sit, Burma

The macro photography (size 3 x 4 cm) of Maw-Sit-Sit shows the finely matted structure of this jadeite-containing rock.

Foto: K. Sieber, www.makrogalerie.de

Maw-sit-site is a rock that consists of six main components that can vary significantly in quantity. The appearance of individual specimens can vary noticeably. This name is derived from a local name of its provenance near the town of Tawmaw in Burma (Myanmar) [Gübelin, 1978].

The rock consists of a groundmass of light-colored aggregates (mainly albite, but also zeolites and chlorite) in composition with numerous dark minerals, especially minerals of the jadeite-cosmochlor solid solution series of intense green color. These jadeite minerals can form single crystals or a blocky mosaic. In between, there are always black nests of fine felted cosmochlore (NaCrSi206), a chromium analogue of jadeite (NaAlSi206).

Oceanchalcedony, Madagascar

Round structures called spherulites are the typical attribute of »Oceanchalcedony«. If they are not present »Ocean chalcedony« can not be recognized as such

Foto: K. Sieber, www.makrogalerie.de

Sphaerolites in Oceanchalcedony

Radiating chalcedony cluster can also connect to agate-like structures

Foto: K. Sieber, www.makrogalerie.de

»Ocean chalcedony« has been known in Europe for a couple of years now. Sometimes the stone is eye-catchingly colorful. It originates in the Mahajanga Basin at the northwest coast of Madagascar. In the Cretaceous period (65 - 130 million years ago) this basin was still a shallow sea basin. About 100 million years ago a tectonic uplift combined with volcanic activities lead to the penetration of hydrothermal liquids rich in quartz into the calcareous and clay sediments. This led to the crystallisation of chalcedony in form of layers and veins.

The primary deposits close to the coast are largely depleted by now. Alternative material from the vicinity usually doesn't have the typical characteristics of »Ocean chalcedony« but contains more jasper and country rock.

The trade name for this variety of chalcedony has changed frequently in the past. Initially the material was offered as "Ocean jasper". This name is still in use in anglophone regions, but it is deprecated. Who ever examines the stone with the loupe, will recognize agate-like structures. In Europe the material was therefore marketed as »Ocean Agate«.

Dr. W. Lieber from Germany microscopically analyzed the stone by means of thin sections. He discovered that the material consists completely of chalcedony that has partially crystallized into radiating, spherulitic structures. Since such structures are called "Spherulites" in mineralogy Lieber named the material »spherulitic chalcedony«.

These round or radiating structures constitute the decisive feature of this kind of chalcedony. They are often embedded into a diversified structured matrix.

Once the mineralogical composition was clarified the trade name »Ocean Chalcedony« became more and more popular. The name »Ocean Agate« can still be used as there are areas with agate every now and then between the spherulitic structures. The name "Ocean Jasper", however, should be avoided. This gemrock does not have a lot in common with jasper. The most accurate way to reference this material is the description »Ocean chalcedony (Ocean agate)«.

For the recognition of a chalcedony as »Ocean Chalcedony (Ocean Agate)« it is essential, that the typical spherulitic structures are clearly present. Only material with spherulites should be labeled »Ocean chalcedony«.

 

Piemontit-Quarzit

Piemontite Quartzite rock, Norway

Image: K. Sieber, www.makrogalerie.de

The best known deposit of »Thulite«, the pink variety of the mineral Zoisite, lies in Norway in the region of Lom. Lom is also the location of piemontite-quartzite which looks very similar to »Thulite« and can easily be mistaken for it.

»Thulite« and Piemontite are Calcium Aluminium Silicates with the same chemical formula Ca2Al3(SiO4)3(OH) (+ Mn, Fe), which crystallize in different crystal systems: »Thulite« (zoisite) in the orthorhombic crystal system, piemontite (klinozoisite) in the monoclin crystal system. When aluminium (Al) is substituted by manganese (Mn), both modifications show pale reddish to strong pink colors. 

eisblumen magnesit

Fig. 1: »Pinolite« - The resemblance to pine nuts gave this magnesite rock its name.

Photo: K. Sieber, www.makrogalerie.de

The Sunk/Hohentauern deposit in Styria (Austria) is known worldwide for a rock forming magnesite with a typical coarsely crystalline growth structure which resemble pine nuts or ice flowers in shape. Because of this special texture the rock was named »Pinolite« (pignolia = pine nut) or "ice flower magnesite". Large, bright magnesite crystals are embedded in a gray, fine-grained matrix rich in inorganic pigment.

Until 1963 the rock was mined as decorative rock and marketed under the name "Sunk" (the name of the finding place). Today it is found almost exclusively in arts and crafts objects or as tumbled stones.

 

»Rainbow Basalt«, USA

»Rainbow Basalt«, USA

Foto: K. Sieber, www.makrogalerie.de

A dark basalt from Michigan (USA) with roundish cavities, filled with colorful minerals has been given the trade name »Rainbow Basalt«.

Basaltic rocks are formed wherever thin, low-silica magma emerges at the earth's surface in volcanically active zones and cools down relatively quickly to basalt lava. Beside very fine-grained plagioclase feldspars and pyroxene minerals, olivine and magnetite may also be present, as well as an uncrystallized glassy component. Sometimes basalt lavas are  quite rich in gaseous components. Therefore roundish cavities are frequently found.

The analysis of some thin sections of the »Rainbow Basalt« showed highly weathered minerals. The initial cavities are now filled with partially complex mineralizations.

Besides feldspar we found calcite and small quartz crystals. To some extent the feldspar is decomposed into clay minerals. In addition, zeolite minerals have been found.

Shattuckite-Chrysocolla-Quartz, Kongo

Shattuckite-Chrysocolla-Quartz, Kongo

Foto: K. Sieber, www.makrogalerie.de

The copper deposit Tantara in Congo is known for its very nice specimens of plancheite and shattuckite with dioptase and white calcite. It also provides a rock composed of Shattuckite, Quartz, Chrysocolla, Azurite, Malachite, Bornite and other copper ores, sold simply as "Shattuckite".

The dark blue copper mineral shattuckite usually occurs as needles or fibrous crystals in radiating or massive mineral aggregats. It is formed as a secondary mineralization in the oxidation zone of copper deposits. With a Mohs hardness of 3½ it is too soft for jewelry.

However, embedded in quartz and together with other copper minerals like azure chrysocolla and/or grass-green malachite it makes up a mineral composition that takes a good polish and is not only pleasant to look at but also an attractive collector's stone.

 

Sodalite-Syenite

Sodalite-Syenite, Brasil

Image: K. Sieber, www.makrogalerie.de

This blue and white speckled stone, which is very popular as a gemrock, has been mined in Brazil since 1965. The name "Azul Bahia", which is commonly used in the stonemasonry industry, refers to this origin.

Petrologically, it is a syenite rock with approx. 50% microclinic feldspar, 30% sodalite, 10% cancrinite, 5% plagioclase as well as 5% aegirine and other dark components. Syenites are magmatic alkaline rocks low in quartz, but often with an exotic mineral composition. Their main mineral composition usually comprises alkali feldspars, amphibole- and/or pyroxenes-minerals or plagioclase feldspars and biotite. In addition, rare minerals such as astrophyllite, eudialyte or (as in in the case of sodalite syenite) sodalite occur in larger quantities.

The mineral Sodalite is closely related to an other blue mineral: lasurite, one of the coloring minerals of Lapis Lazuli rock. Contact with (even weak) acids should be avoited, as it can turn grey under their influence.

Sodalite-syenite is used as a decorative stone (gemrock) in the jewellery industry, where it is also known as "Sodalite" abbreviated.

 

 

»Sonora Sunrise«, Mexico

»Sonora Sunrise«, Mexico

Foto: K. Sieber, www.makrogalerie.de

This colorful stone comes from the Milpillas Copper Mine west of the small town Cananea in the North Mexican Sonoran Desert. The mine is small and produces not very much material. As a consequence the stones achieve high prices ever since their market launch in 2006.

Depending on the mineral composition the color varies from brownish red with olive-green spots, to black areas with a metallic luster, together with blue and green areas, grown closely together like a patchwork. Using X-ray diffraction analysis (EDX) the green-blue zones could be identified as an intergrowth of  brochantit (a copper sulfide) with the green copper silicate chrysocolla. The brownish to orange-red areas consist of the copper oxide cuprite grown together with another copper oxide: tenorite.