Quartz – so many gems, so much confusion!

Quartz, chalcedony, agate, jasper … What’s the difference?

The quartz and chalcedony gems provide a wide range of beautiful colors and patterns with very good durability. Although they all have the same basic chemical formula, SiO2, different impurities and/or conditions when the minerals form create different end results.

First we’ll define two major groups based on crystal size: quartz and chalcedony (pronounced kal-sed-n-ee).

Quartz varieties have crystals that can be seen with an optical microscope (and sometimes with the naked eye).

AMSSN102_a

Amethyst (item AMSSN102)

Some quartz varieties commonly used in jewelry include:
Amethyst – purple
Ametrine – purple and yellow in the same stone
Citrine – yellow
Rock crystal – colorless
Rose quartz – pink
Smokey quartz – brown

Chalcedony varieties are cryptocrystalline which means that individual crystals cannot be seen, even with the use of an optical microscope.  Chalcedony gems are subdivided into two groups: agate and jasper.

Agate has angular or curved banding. 

Blue lace agate  (item BASSN101)

Blue lace agate
(item BASSN101)

Some agates used in jewelry include :
Blue Lace Agatelight blue bands in a lacy or wavy pattern
Botswana Agate banded with fine parallel lines of white, purple, or peach
Crazy Lace Agatetwisting and turning bands of various colors
Eye Agatebanded, concentric rings that are perfectly rounded
Fire Agate  –  Form of Agate or Chalcedony that is iridescent

Jasper is a “general variety term for opaque chalcedony of any color or combination of colors except solid black or specially named material” (GIA).

Mixed jasper necklace (Item JSGFN101) displayed on ocean jasper slab

Mixed jasper necklace (Item JSGFN101) displayed on ocean jasper slab

Examples of “specially named” varieties include:
Bloodstone – dark green with red spots
Carnelian – orange
Chrysoprase – apple green
Onyx – banded, black and white
Sardonyx – banded, brown/red and white

To add to the confusion, a number of gems don’t follow the naming conventions:
Moss agate – green inclusions, not banded
Onyx- banded, should be agate
Sardonyx – banded, should be agate

There are also many quartz minerals that are not beautiful or rare enough to be considered gems, such as chert and milky quartz.

With all these beautiful colors and patterns to choose from, there’s a quartz or chalcedony gemstone for everyone! Which is your favorite?

Information sources:
Gem Identification Lab Manual, Gemological Institute of America, 5/2012
http://www.minerals.net
http://www.wikipedia.org

Amethyst: Power to the Purple!

amethyst

Amethyst crystals, cut gem, and beads

February’s birthstone is amethyst, the purple variety of quartz.

Last month I told you about the many different colors of garnets, which are caused by differences in chemical formula. Pure quartz (chemical formula SiO2) is colorless (and is called “rock crystal” in the jewelry industry). The different colors of quartz* are caused by defects in the crystal structure rather than by a consistent change in the  chemical formula. This often causes different amounts of color within even a single crystal (“color zoning”). For amethyst, iron, and possibly aluminum, impurities are believed to cause the purple color.

Amethyst deposits are found all over the world; many of the large mineral specimens come from Brazil and Uruguay, but other sources for the gem include Russia, Czechoslovakia, Zambia, and the United States. It is not uncommon to find relatively large crystals which makes this a very affordable gem, even in large sizes.

Other facts – Amethyst:

  • is the gemstone for those celebrating their 6th or 17th wedding anniversary
  • was believed to prevent intoxication—amethystos means “not drunk” in ancient Greek
  • is sometimes found in combination with citrine and is then known as “ametrine”
  • when heated can change color to become a golden citrine
  • has a hardness of 7 on the Moh’s scale

*Other quartz gemstones include citrine (yellow), aventurine (green), rose quartz (pink), tigers eye (brown), smokey quartz (brown or gray) and chalcedony (which includes jaspers and agates).

Sources:

  • Gemological Institute of America, Gem Encyclopedia page on amethyst: click here.
  • Manual of Mineralogy, 20th Edition, Cornelis Klein and Cornelius S. Hurlbut, Jr., 1977.
  • GemSelect