Current research results on precious stones and gem rocks.
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At the beginning of 2020, a pink mineral with an attractive texture similar to the pectolite variety "larimar" was found in veins and fissures of a local basalt on the Indonesian island of Nusa Kambangan (Central Java). The stone was promptly marketed under the trade name "pink larimar". Other sources on the Internet state that the zoned mineral aggregates are the mineral thomsonite. The EPI-LAB had the opportunity to examine this gemrock more closely.
Fig. 1: Nundorite - an metamorphically altered phonolitic volcanic rock
Photo: K. Sieber, www.makrogalerie.de
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There are not only different names and spellings for this rock from Australia in the literature and in the trade, but also different places where it was found. It is questionable whether all these informations mean exactly the same rock.
The rock examined in the EPI laboratory comes from New South Wales in Australia. It shows light green, cloudy areas embedded in a pinky-beige to brown matrix (Fig. 1). Small particles of opaque ore minerals are unevenly distributed throughout the rock.
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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 by using our microscope 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 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«.
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.
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.
