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Roger Weller, geology instructor                                         

wellerr@cochise.edu
Quartz Family Gemstones-by Audrey LaClair
                            
                          Ametrine

           Ametrine is quite rare in that it is a combination of amethyst and citrine in one quartz.  So in turn, it is a beautiful half yellow and half purple gem.  It is caused by iron in the crystalline structure undergoing different oxidation states.  "For ametrine to occur, iron impurities in the Fe⁴⁺ oxidation state must be present in the specific color producing sites in a portion of the quartz and Fe³⁺ must be present in the specific color producing sites in another portion of the quartz. There are a number of processes by which iron can be present in more than one oxidation state in different parts of the same specimen, and they are not all fully understood.  Both amethyst and citrine are thought to have been crystallizing simultaneously during formation. For this to occur, the temperature would have to be near to the point above which Fe³⁺ would enter the appropriate color producing site in quartz thereby forming citrine, and below which Fe⁴⁺ would enter the appropriate site resulting in the formation of amethyst. There had to be a slight difference in temperature between the crystallizing surfaces of the quartz, so that some of the surfaces were of slightly higher temperature and thereby crystallizing as citrine while other surfaces were slightly cooler and crystalizing as amethyst. This could occur if one side of the crystal was facing a vent or other such heat source. Any change in pressure would affect the temperature at which citrine formed instead of amethyst. This delicate balance of the temperature, pressure and chemical environment of the crystallizing ametrine quartz all had to be maintained while crystallization proceeded.  Ametrine is readily recognized by its colors, hardness, glassy luster, conchoidal fracture and lack of cleavage" (http://www.mineralminers.com/html/amtminfo.htm)

photo by R.Weller

          Ametrine is also rare because it only comes from one mine.  It comes from the Anahi Mine in Bolivia in the rainforest.  It is amazing how inexpensive ametrine is considering it only has one source!  Ametrine was not on the market until 1980 when the Anahi Mine sold it commercially.  Before that, it was a rare commodity.  Although, several suppliers have said that the ametrine mine has run out and that quality material is very hard to get.
 

photo by R.Weller

PHYSICAL PROPERTIES

·        Ametrine Chemical composition: SiO₂ (minor iron impurities cause ametrine's colors)

·        Class: tectosilicate

·        Crystal system: Hexagonal-R; 32 (trigonal-trapezohedral)

·        Crystal habit: Macroscopic crystals occur as horizontally striated hexagonal prisms terminated by a combination of positive and negative rhombohedrons forming six sided pyramids. It can also be massive.

·        Twinning: Dauphine twin with c the twin axis, Brazil twin with {1120} the twin plane

·        Specific gravity: 2.65

·        Index of refraction: 1.54-1.55

·        Birefringence: 0.009

·        Pleochroism: distinct

·        Hardness: 7

·        Color: two distinct colors present, yellow-orange and purple

·        Luster: vitreous

·        Transparency: transparent to translucent

·        Cleavage: none

·        Fracture: conchoidal

·        Streak: white

From (http://www.mineralminers.com/html/amtminfo.htm)
 

References:

http://www.gemstone.org/gem-by-gem/english/ametrine.html
http://www.generousgems.com/ametrine.html
http://www.ametrine.com/home.htm
http://www.gemhut.com/ametrine.htm
http://www.mineralminers.com/html/amtminfo.htm