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Ch. 3: Beryl: Emeralds, Aquamarine

Ch. 2: Corundum: Sapphires, Rubies Page of 96 Ch. 3: Beryl: Emeralds, Aquamarine Text size:minus plus Restore normal size   Mail page  Print this page
 
     
     
 
GEMS AND PRECIOUS STONES. 27
  
 
 
  
 
If sis planes are placed parallel to the principal axis of the hexagonal pyramid, allowing the lateral axes to join the edges (if the first order), or the centre of the planes (if the second order), and closing the figure with two hexagons, we get a form called a hexagonal prism. This form is bounded by six rectangles and two regular hexagons, or the ends of the prism may be closed by other forms in combination ; a hexagonal pyramid, for instance, as is often seen in quartz crystals.
The dihexagonal pyramid has double the number of faces, or twenty-four equal isosceles triangles, if the form is a compound of the first and second orders of pyramids.
The dihexagonal prism has twelve rectangular faces and the basal planes are dodecahedral. This form has been identified with corundum.
These are the principal holohedral forms belonging to the hexagonal system.
There are two hemihedral forms belonging to this system often found in minerals; the most important is the rhombohedron. This is formed by extending each alternate face of the hexagonal pyramid to the exclusion of the others. The figure may be either obtuse or acute, this depending on the original form of the pyramid. It is bounded by six rhombic planes, and the principal axis joins the two equal three-sided solid angles.
The hexagonal scalenohedron is the hemihedral form of the dihexagonal pyramid. It is formed by the suppression of alternate pairs of faces, and not alternate faces as in the rhombohedron ; it is bounded by twelve scalene triangles.
This system is a very beautiful and complicated one, although the numerous combinations do not interest us in the study of precious stones.
It should not be difficult for anyone to distinguish gems crystallizing in the above forms of the hexagonal system, although it is hardly to be expected that natural gems will give the perfect symmetrical forms as illustrated above. When found in alluvial deposits the edges of the crystals are usually more or less worn or abraded.
BERYL,
and its Varieties, Aquamarine and Emerald.
  
 
 
  
 
 
 
  
 
It is one of the few minerals which occurs only in crystals, and that has no essential variations in chemical composition.
Crystals have been found of enormous dimensions. One shown at the London Exhibition of 1851 weighed 80 lb., but this must be considered quite small in comparison with some crystals found at Grafton, New
 
 
  
 
 
  
     
Ch. 2: Corundum: Sapphires, Rubies Page of 96 Ch. 3: Beryl: Emeralds, Aquamarine
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