Calcite "dogtooth". Chihuahua,Mexico.

Calcite "dogtooth". Chihuahua,Mexico.

Calcite,   CaCO3   This specimen of "dogtooth "calcite is from the Santa Eulalia mining district, Chihuahua, Mexico. The crystals are grey in colour, many with lustrous surfaces, which implies colouration by impurities rather than surface coating. There is only one damaged crystal, the cross section of which shows interesting markings. Many of the dog tooth crystals are coated with very small irregular crystals of calcite, and the rear of the specimen is about 50% covered with microscopic druzy calcite.

Sphalerite, Nentsberry Haggs.

Sphalerite, Nentsberry Haggs.

Sphalerite. ( Zn,Fe ) S ),  Zinc sulphide, with variable iron content. Sphalerite is the main ore of zinc, and sometimes referred to by miners as zinc blende, or black jack. This specimen is an agglomerate of loosely cemented pieces of rock with nearly all of the surface, and all crevices, coated with small crystals of sphalerite . The crystals on the rear of the specimen are slightly larger than those on the front. The location is from the "Second Sun" vein, Nentsberry Haggs mine, Cumbria.  

Gypsum, selenite var. South Australia.

Gypsum, selenite var. South Australia.

Gypsum, CaSO4 . 2H2O is an evaporite mineral. This selenite variety specimen is composed of numerous sprays of small tabular crystals, varying in colour from clear to light green. The location is from the Pernatty lagoon, near to Mount Gunson copper mine in South  Australia. The green colour is attributed to copper from the nearby mine.

Pyrite, Navajun,Spain.

Pyrite, Navajun,Spain.

This beautiful crystal of pyrite, FeS2  is from a locality renowned for an abundance of such specimens. The locality is La Riorga, Navajun, Spain. There are several smaller crystals on the reverse of this specimen, which are cubes, but the main crystal is not a true cube, but is elongated, which is a little unusual for specimens from this location. The crystals were probably formed during the "diagenesis" stage of carbonate muds , which later lithified into the marl matrix in which the crystals are now embedded. The crystals easily fall out of the matrix during preparation, and are replaced by dealers, however, all the crystals in this specimen are in their original placement, and there are obvious marks on the matrix which show the sites of  lost crystals.    

Quartz ( Eisenkiesel var. ). Pendeen.

Quartz ( Eisenkiesel var. ). Pendeen.

Eisenkiesel is a German term which translates to iron quartz, or ferruginous quartz. In this specimen the pyramidal crystals of quartz are step faced, with a dark grey/brown colour, and are partially covered with "paper calcite" crystals. The specimen is from the coronation lode of the Geevor mine, at Pendeen in Cornwall.