Quartz and its colored varieties


For information on other polymorphs of silica, see http://minerals.gps.caltech.edu/Silica_Polymorphs/index.html


  Quartz mine in Minas Gerais, Brazil


Iron-containing varieties:

Amethyst

Large geodes of amethyst occur in basalts in southern Brazil and northern Uraguay.

The color of amethyst is the result of radiation damage to Fe3+ in the interstitial site of quartz.  The radiation could be due to gamma rays from 40K.  Current theory says that the ferric iron is oxidized to Fe4+ by the gamma rays.


Amethyst geodes in basalt near the town of Ametista do Sul, Rio Grande do Sul, Brazil
 


Amethyst geodes and citrine geodes (heated amethyst ) for sale near Rio de Janerio, Brazil.

Amethyst geode from Rio Grande do Sul, Brazil


Citrine

Most citrine is made by heating amethyst.  Typically low-quality amethyst is heated to develop the citrine color.
 


Synthetic Russian citrine is produced directly by the addition of iron to the growth solutions.  It owes its color to Fe3+ clusters in the quartz.


Green quartz

One of the rarities of nature, green quartz forms when amethyst from certain deposits is heated to a critical temperature.  The color is from Fe2+ in the approximately octahedral interstitial site.  Green quartz can be made synthetically by adding Be2+ to the tetrahedral site which allows more Fe2+ into the interstitial site from the coupled substitution:  Si4+ (td) = Be2+ (td) + Fe2+ (int).

Here is natural green quartz which resulted from an amethyst deposit being overrun by a basalt flow.  This is from Thunder Bay, Canada. Specimen provided by SA Kissin.
 
 


Synthetic green quartz with Fe2+ in the interstitial site.


Ametrine

Ametrineis bi-colored variety of quartz containing both amethyst and citrine zones
 
 This slice and crystal are from the only significant source in the world, the Anahí Mine, Bolivia.
Much more information on ametrine can be found at this link.


Smoky quartz

 The result of radiation damage to Al-containing quartz.

The crystal on the left looked like the one on the right before it was exposed to about 10 Megarads of gamma radiation from 137Cs.
 


Rose quartz (common, massive rose quartz)

Click here for more information on rose quartz. The color of common, massive rose quartz is from sub-microscopic fibers of a phase related to dumortierite.
 
 
Asterism (star reflection of light) in rose quartz.  The reflection comes from the fibers which are partially ordered along the a-axes of the quartz.  Photo courtesy of M Scott A scanning electron microscope image of the fibers extracted from rose quartz.  The individual fibers are about 0.1 mm wide.  The false color in the image is close to the true color of the fibers. SEM photo courtesy of J Goreva, Caltech.

Pink quartz (uncommon, crystalline "rose" quartz)

There is an rare type of pink quartz (also frequently called crystalline rose quartz) which differs from the abundant massive rose quartz found throughout the world.  It is found in veins up to about 6 cm wide of rose colored euhedral crystals.  This material occurs in phosphate pegmatites near Galiléia (near Gov. Valadares), Minas Gerais, and a few other localities in Brazil and elselwhere.  It is photosensitive; the color of the natural crystals fades in light and can be regenerated by exposing the crystals to ionizing radiation. The color mechanism has been proven in the synthetic material.  It is synthesized by growing quartz in the presence of aluminum and phosphate, and exposing the product to gamma rays.
Al - O2- - P   +  g-ray   ®     Al - O- - P    +   electron

Pink quartz crystals from Minas Gerais, Brazil. 
Picture courtesy of S. Pitts

A slab of synthetic Russian pink quartz grown with Al and P substitutions and subsequently irradiated with gamma rays to develop the deep rose color. 
 
 
 

Specimen courtesy of V Balitsky


Blue quartz

Natural blue quartz contains sub-microscopic inclusions of ilmenite or larger inclusions of other phases.
 
 
These are inclusions in 'blue quartz' from Madagascar.  The ~0.1 mm inclusions are probably in the lazulite-scorzalite series. This blue-grey quartz from Nelson, Virginia, contains micro inclusions of ilmenite. TEM photo courtesy of Ma Chi, Caltech

 


Chrysoprase

The color of chrysoprase is due to minute inclusions (98K) of nickel silicates in the silica. In the case of the Australian chrysoprase, the nickel silicate is a member of the talc group which been identified both as willemseite, (Ni,Mg)3Si4O10(OH)2, and as kerolite, a variety of talc with a randomly stacked structure, in the series, kerolite - nickel-kerolite.


Colorless quartz

 Colorless, synthetic quartz grown by the Western Electric Corporation for use in communication devices.
 
A large group of colorless quartz crystals from Hot Springs, Arkansas. 
 
 
 
 
 
 
 

Picture courtesy of Jewel Tunnel Imports.


Agate


 This agate slice is from Rio Grande do Sul, Brazil.  The agates occur in the same basalts as the amethyst geodes.  This is a slice in its natural state.  Many agates are dyed or treated to change their colors.


Fluid Inclusions

This is a smoky quartz crystal from Minas Gerais, Brazil, with a large
fluid inclusion (water) with a large gas bubble above the water.