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Roger Weller, geology instructor                                         

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

         Rock crystal is colorless and transparent, the name is given to all clear and colorless quartz.  It's use is mostly for ornamental stones, but occasionally used as a gemstone to imitate diamonds.  The crystal usually forms in the cavities in quartz-veins. Radiation will turn rock crystal in a brown or gray color, know as smoky quartz.  It was once believe that rock crystals were permanently frozen ice.  Rock crystal is cool to touch, so this added to their beliefs.

photo by R.Weller
 

         In the 1980's and 1990's, rock crystal became very popular for metaphysical uses and applications, about 40% of rock crystal was used for this.  A good example would be the crystal ball, but that has been around much longer than that.  Today, the metaphysical use for rock crystal is not as prominent, about 15-20% and the market has stabilized.

         "Rock crystal is recognized by its crystal habit, transparency, hardness, glassy luster, conchoidal fracture, occurrence and general lack of cleavage" (http://www.mineralminers.com/html/rkxminfo.htm)

         One can find rock crystal in Brazil, where large individual crystals can be found, the largest one weighs in at over 44 tons.  You can also find it in England, Madagascar, Switerzland and Hot Springs, Arkansas.

photo by R.Weller
 

PHYSICAL PROPERTIES
 

·        Rock Crystal chemical composition: SiO₂

·        Class: tectosilicate

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

·        Crystal habit: Macroscopic crystals commonly occur as horizontally striated hexagonal prisms terminated by a combination of positive and negative rhombohedrons forming six sided pyramids. Prism faces and/or rhombohedral terminations may be lacking, poorly developed or predominant resulting in diverse possible crystal habits. Trigonal trapezohedral faces can occur in the upper right or left of alternating prism faces identifying right or left handed crystals, respectively.

·        Twinning: Dauphine twin with c the twin axis, Brazil twin with {1120} the twin plane, Japanese twin is rare with {1122} the twin plane.

·        Specific gravity: 2.65

·        Index of refraction: 1.54-1.55

·        Birefringence: maximum of 0.009

·        Pleochroism: none

·        Hardness: 7

·        Color: colorless

·        Luster: vitreous

·        Transparency: transparent

·        Cleavage: none

·        Fracture: conchoidal

·        Streak: white

·        Electrical properties: strongly piezoelectric and pyroelectric

From http://www.mineralminers.com/html/rkxminfo.htm
 

References:

http://mineral.galleries.com/minerals/gemstone/rock_cry/rock_cry.htm
http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/quartz.html
http://23.1911encyclopedia.org/R/RO/ROCK_CRYSTAL.htm
http://www.mineralminers.com/html/rkxminfo.htm