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Azurite-Gneis, Pakistan

Azurite-Gneiss, Pakistan

Foto: K. Sieber, www.makrogalerie.de

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The rock that we are presenting to you looks as if it had been decorated with blue inkblots. It was presented to us as "azurite granite". It is reported that it is found at the foot of the K2 mountain in northern Pakistan. Therefore it is also offered under the trade name "K2".

The structures we saw in our microscopic investigation was similar to that of gneiss. Quartz, biotite and feldspar crystals and a weakly pronounced, layered structure were easily recognizable. In addition, small grains of a green mineral and these strange blue spots are readily visible with the naked eye.

The carbonate test with 10% diluted hydrochloric acid showed an unspecific reaction pattern. The green mineral reacted promptly to the acid, classifying it as malachite. In the rest of the rock a weak but irregular reaction could be observed, no matter whether these domains were green, blue or colorless. It seemed as if carbonatic minerals had penetrated the whole stone.

The occurrence of basic copper carbonates such as azurite and malachite (which are typical for sedimentary deposits) in a metamorphic gneiss is highly unusual. Therefore, we decided to clarify the mineral composition by means of X-ray diffraction analysis (EDX).

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»Budstone« (Greenstone) - 4.9 out of 5 based on 59 votes
Budstone is NOT Prasem

Budstone, South Africa

Foto: K. Sieber, www.makrogalerie.de

The inhomogeneous appearance of a green rock marketed as »Prasem« from South Africa raised doubts about its authenticity. EDX (Energie Dispersive X-Ray) and Raman Laser analyzes performed on some samples confirmed a rock with an inhomogeneous complex mineralogical composition.

Analytical investigations

To our surprise, the X-ray diffraction analysis (EDX) of a sample with particularly hard, dark green to black inclusions reveiled the rare mineral eskolaite together with rutile and chromium bearing mica (fuchsite). Eskolait occurs only at 11 locations worldwide (e.g. Brazil, USA, Russia, India and Finland). Its hardness of 8½ is between spinel and corundum. On the other side the mineral fuchsite has a hardness of only 2½. The resulting differences in hardness within the rock lead to bud-like, roundish weathering forms, which inspired the trade name »Budstone«.

In other samples we found large amounts of plagioclase feldspar and the weathering products kaolinite and clinochlor in small quantities. These findings could be confirmed by Raman laser analyzes. This mineral paragenesis points to a metamorphic rock, the green color of which is caused by phyllosilicates (fuchsite, clinochlor). Such rocks are called »greenstones« in geology.

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»Dalmatian Stone« is NOT a Jasper

»Dalmatian Stone« is NOT a Jasper

Foto: K. Sieber, www.makrogalerie.de

This speckled rock in black and white, which was named after the Dalmatian dog breed, was long thought to be a porphyrite (a volcanic rock). Also other speculations about the mineralogical composition and formation of this rock, ranging from a volcanic porphyrite to a rock with black tourmaline, were not scientifically proven.

Reason enough for the EPI laboratory to take a closer look at this rock. Even the first analyses of the microstructure led to the assumption that this stone could be a gangue rock. To verify this assumption, it was necessary to precisely determine the mineral composition of this rock. Of particular interest was the way of formation and the nature of the black spots. Therefore we first tried with Raman laser analysis to find out what these black spots consist of. The analyses proved that the black spots cannot be tourmaline, but a mineral from the amphibole group. Since this group is extremely diverse and has numerous solid solution members, no precise conclusions could be optained from the Raman spectrum.

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Eilat-Stone

»Eilat Stone«, Israel

Image: K. Sieber, www.makrogalerie.de

From time to time a trade name traditionally used as a quality label is retained, even when the original qualities are no longer available. Often this name is simply transferred to other stones or color varieties that look roughly similar to the original. This is also the case with the so-called »Eilat-Stone«, which appears both on the Internet and on home shopping television.

The classic occurrence of this colorful stone is 25 km north of the city of Eilat, on the southern tip of the State of Israel. In the Timna valley, on the edge of the Negev desert, copper ore was mined over 5000 years ago. The legendary copper mines of King Solomon, from which most of the turquoise used in ancient Egypt originate, are located in the same area.

»Eilat-Stone« is composed of a variety of copper minerals, especially carbonate minerals (malachite, azurite), phosphate minerals (pseudomalachite, turquoise) and silicates (chrysocolla, quartz). Depending on how these components are distributed, blue, blue-green or green colors dominate. These copper mineralizations are formed in a magmatic complex of predominantly granitic composition, which is about 550 million years old. The magmatic activity produced copper-bearing solutions from which copper minerals crystallized, leading to complex mineral aggregations in the oxidation zone.

Since the site was declared a national park, the original »Eilat-Stone« is no longer available. It has been replaced by chrysocolla-malachite from Peru, which is rich in country rocks. Although they also contain the carbonate and silicate minerals of the original »Eilat-Stone«, they lack the phosphate minerals. What sometimes looks like light blue turquoise in the Peruvian stones is chrysocolla, which can be easily checked by its ability to stick to the tongue in its raw, untreated state.

So if you are offered a genuine »Eilat-Stone«, you should pay special attention to the presence of phosphate minerals, which are a good authenticity feature.

 

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»Dumar«, China

»Dumar«, China

Foto: K. Sieber, www.makrogalerie.de

The origin of the name »Dumar« is unknown. Whether the name has anything to do with the Spanish. Mar = sea is speculative. As origin "China" is indicated.

Like many other newly discovered rocks, »Dumar« was initially called "Jasper" or "Lotusjasper". But the rock has nothing to do with the original jasper. Mineralogical investigations have shown that it's a very complex skarn rock. Skarns are metamorphic rocks formed by metasomatosis from limestone or dolomite rocks.

The Dumar-Skarn consists of a silicified matrix with very fine-grained tremolite, partially intergrown with dolomite and calcite. Additionally radial grown augite and aegirin-augite occur in larger crystals. In the lighter areas they are intergrown with albite feldspar. In between violet domains of fluorite occur, which forms in some regions idiomorphic crystals.