Opal, Andamooka.

Opal, Andamooka.

Opal,  SiO2 . nH2O  is a hydrated amorphous form of silica, with a water content between 6% and 10%.  Opal is non-crystalline, so it is classed as a mineraloid. Conditions during deposit can determine the gem quality of opal, for instance, if there is undisturbed deposition of identical sized minute spheres of silica, gem quality opal is formed.  There is only a hint of "pinfire opal" in the green area of the specimen which was collected in Andamooka, Australia, by the club co-founder, Jim Nicholls.    

Blue John,(Fluorite). Treak Cliff cavern,

Blue John,(Fluorite). Treak Cliff cavern,

Blue john is a variety of fluorite, CaF2 Calcium fluoride. The type locality for this mineral is Castleton in Derbyshire, and this specimen came from Treak Cliff cavern in Castleton. Although Blue John is mostly  associated with Castleton, and is sometimes referred to as Derbyshire spar, similar deposits have been discovered in China.

Grossular ( garnet ), Grova.

Grossular ( garnet ), Grova.

Grossular,  Ca3Al2(SiO4)3   is a calcium-aluminium species of the garnet group. The colour of grossular is very variable, dark green, pink, brown, even colourless, and it's crystal classification is cubic, although most crystals are hexoctahedral.  The specimen is from Grova in Southern Norway, an area known for copper mining. The crystals are cubic, and a very dark brown. They have slightly bevelled edges, and there is a quartz infilling of a fracture which runs through some of the crystals.  

Chalcedony, Normandy.

Chalcedony, Normandy.

Chalcedony,  SiO2   is a cryptocrystalline mineral formed from the intergrowth of two silica minerals, quartz and moganite.  The specimen, from Normandy in France is in the form of part of a geode. The outer wall of the geode exhibits the conchoidal fracture associated with Quartz. and the interior is lined with mammarian growths, of a pinkish hue, which are soapy to the touch.

Gypsum, Nottingham.

Gypsum, Nottingham.

Gypsum, CaSO4 . 2H2O  is an evaporite mineral, commonly found in layered sedimentary rocks. The specimen is a selenite variety of gypsum known as " Satin Spar " It is 9cm. x 9cm. as per view and 7 cm. deep. Note the faint centerline  where the fibrous gypsum has grown inwards from two opposing walls to meet in the center. The location is loosely referred to as Nottingham