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).

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)

Some agates used in jewelry include :
Blue Lace Agate – light blue bands in a lacy or wavy pattern
Botswana Agate – banded with fine parallel lines of white, purple, or peach
Crazy Lace Agate – twisting and turning bands of various colors
Eye Agate – banded, 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

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