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A study of  MADAGASCAR CORUNDUM
 AL2O3

Sapphire found in a metamorphic marble.
· Crystallographic properties of corundum: 
Trigonal.c = 1,3630     pp 0 86°4' ; pa1 = 122°26'
Macle according to p (1011), often polysynthetic, 
accompanied by plans of separation,  similar plans are also
observed according to a1 (0001) unequal break to conchoïdal.
Hardness 9. Density 3,95 to 4,1  Refractive index: 1.76 - 1.77
Uniaxial and optically négative.ng = 1,7675; Np = 1,7593
The faces a1 frequently show the phenomenon of asterism,
generally due to reflexions within separation p.
Strong polychromatism , with following maximum ng.
· Chemical properties:  corundum is composed of pure
alumina; its colouring is due to metallic oxide traces or inclusions.   The story
The color of sapphire, due to iron and titanium oxide, gives its
best effect under daylight.  Electric light makes it often dark.
The color of the ruby, due to chromium oxide, on the contrary shows 
its most luminous red under electric lighting.
         
   In Madagascar, corundum shows two different aspects:
  1  stony and opaque crystals.
  2  crystals of smaller size often transparent and usable as gem.
  In Madagascar, stony corundum comes from mica schists metamorphosed by granite, as well as
  endomorphosed granitic veinules  and more or less deprived of quartz which injects those. Silimanite is 
  an usual satellite of corundum in this type of deposit. One also finds some one in eruptive rocks, 
  syenites. 
  Gems are generally found in alluvium, but occur either from basaltic slags, or from metamorphic 
  marble or endomorpheous feldspatic rocks.
1 2 3 4 5
One distinguishes two principal types of corundum in Madagascar:
Type I : isoceloedric, more or less acute accompanied or not by a small face a1 (0001) and more rarely 
              by facets p (1011). Represented by figures 1 to 11, but which often become complicated in  
              consequence of the irregularity of the development of some of their faces and by stacking with 
              parallel axes of a great number of  individuals.
    
6 7 8 9 10
     
11 12  

13
     Scalenoedron          Stacking with parallel axes,   Crystal supporting on one of its bases
        gutters at contours of the face.   a small rhombohedron p in parallel 
  position.
 Type 2: Characterized by the association of the prism d 1 (1120) at a broad base, with which can 
              associate the isosceles ones, among which e 3 is most frequent, as well as the rhombohedron p.
              The base of  Malagasy corundum crystals very frequently shows scratches or triangular figures in 
               relief, limited by p.
Twin Sapphire crystal, blue Madagascar mineral, exclusive sapphires, corundum information data
Macle of blue corundum.
                   Violet pink tabular Sapphire crystal, blue pink Madagascar mineral, exclusive sapphires, corundum information data                      
Translucent violet - pink sapphire
              Ruby crystal in amphibolite, purple Madagascar mineral, exclusive crystals, corundum information data
Ruby crystal, red Madagascar mineral, exclusive rubies, corundum information data
                             Ruby in amphibolite.
 The crystals engaged in rocks, rich in mica, have rough faces encrusted of biotite or muscovite, minerals
 which also exist at the state of inclusions, associated with magnetite and pyrite.
 Because of its hardness, its inalterability and its density, corundum constitutes a standard mineral of 
 alluvia. 
Big ruby crystal, red Madagascar mineral, exclusive corundums, corundum information data Huge Andranondambo Sapphire crystal in metamorphic marble, blue Madagascar mineral, exclusive sapphires, corundum information data
                     Ruby                Giant Sapphire in a metamorphic dolomitic marble.
Iankaroka polychrome Sapphire crystal discovered in 1990 by Alain Darbellay, padparadscha Madagascar mineral, exclusive sapphires, corundum information data Andranondambo Sapphire crystal, blue Madagascar mineral, exclusive sapphires, corundum information data Ilakaka Sapphire crystal, blue Madagascar mineral, exclusive sapphires, corundum information data
 Polychrome Sapphire   Blue Sapphire ( origin:  Blue Sapphire ( origin: Ilakaka 1999 )
 ( origin: Iankaroka 1990 )  Andranondambo 1992 ) Found in the sandstones of Isalo in
 This sapphire comes from a granit,  The stone comes from  alluvium, the shape is rounded.
 its shape is intact.  a metamorphic marble. The origin of the stone is a marble.
· The structure of crystals.
It is R.J.Haüy who put forth the first assumption, confirmed 200 years later, on the structure of the 
crystallized material. He lets drop accidentally a calcite crystal which broke into small rhomboedrons. He 
supposed whereas the crystals were consisted of stacking small rhomboedrons elementary (that he called 
the "integral molecule"), he could reconstitute all known shapes of calcite. In a crystal lattice, three atoms 
or groups of atoms (nodes), which is not on the same line, determine a plan called reticular plan. But there 
is a great number of these plans: it is enough to choose nodes not strictly nearby. As the network is infinite, 
there is even an infinity of possible plans. The dimension of the faces does not play any role. During its 
growth, a face can develop more quickly than another. The angle that the faces make between them 

remains constant, since it depends on the network. This structure of the crystalline material explains the 
law of the constancy of the dihedral angles.
White Sapphire crystal, uncolored Madagascar mineral, exclusive sapphires, corundum information data White Sapphire crystal with triangle paterns, uncolored Madagascar mineral, exclusive sapphires, corundum information data Stacked ruby crystal, red Madagascar mineral, exclusive rubies, corundum information data Translucent ruby crystal, red Madagascar mineral, exclusive rubes, corundum information data
Triangular figure on a Scratches on a rare Stacking in macle  Triangles on a translucent
colorless corundum. colorless corundum. of a ruby. corundum.
    
Triangular Andranondambo blue corundum, blue Madagascar mineral, exclusive sapphires, corundum information data Twin ruby crystal, red Madagascar mineral, exclusive rubies, corundum information data
    Triangular figure on a     Triangles in step on a blue   Remarkable macle of
    blue corundum.      corundum from Maromby.   red corundum.
               
Sapphire & Ruby Safari Gem ruby from Ambatovaravarana.
                                     The alluvial ruby deposit of Ambatovaravarana in the highlands, 
                                      is located in the center on the picture, under the hill.                        ©  Image J. Darbellay
Macle of blue sapphire.      Macle of ruby 60 grms.
Information ¦ Cutting Styles ¦ Characteristics ¦ Crystalline Systems ¦ Madagascar Sapphire
 Andranondambo Sapphires  ¦  Blue Sapphire Crystal  ¦  Ruby Crystal ¦ Sapphire Slice
   
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© All of the pictures on this website have been shot by gggems.com Alain Darbellay
     Text written by Alain Darbellay.
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