As I stated in my last blog post, quartz can be considered the head of a family containing chalcedony, agate and jasper to name a few. But, I also find information that says agates are a chalcedony. I put the confusing question to the experts at my rock forum and came up with a clear definition. I received an outstandingly simple answer to my complicated and confusing question.
Quartz comes in four forms, one being microcrystalline. The microcrystalline variety of quartz is called chalcedony. Chalcedony is the parent of agate, jasper and jasper agate.
Colorless chalcedony is pure microcrystalline quartz. It can be colored by mineral impurities, and will usually be translucent but will have no pattern or banding. Often, chalcedony will have crystalline impurities which we call dendrites, plumes and moss. (They are usually named agate, but are actually and technically chalcedony. ) A chysoprase is a true chalcedony. A Bloodstone is a dark green included chalcedony.
|Chrysoprase marked out to be cabbed.|
|Bloodstone (Heliotrope) A dark green Chalcedony with red iron inclusions.|
Agate is a banded, translucent to semi translucent chalcedony (or banded microcrystalline quartz). A good example would be a Brazilian agate.
|A banded Brazilian agate slab with a ghost face.|
Jasper is an opaque chalcedony (or opaque microcrystalline quartz). Willow Creek Jasper is a good example of jasper.
|Willow Creek Jasper|
Jasper agate is an opaque stone with semi transparent to transparent areas of chalcedony or agate (or semi opaque microcrystalline quartz). Stone Canyon Jasper would be a good example of jasper agate.
|Close up of some Stone Canyon Jasper rough|
So, it is true that chalcedony, agates and jaspers are quartz. But, they are microcrystalline quartz and a microcrystalline quartz is called chalcedony.
The real confusion comes from the different rocks having names that bear no resemblance to what they actually are.
For instance, Amethyst Sage Agate is actually a purple chalcedony with crystaline impurities (inclusions) which form black dendrites.
|Amethyst Sage Agate (Purple Chalcedony with dendrite inclusions)|
A Moss agate is actually a clear chalcedony with crystaline impurities called moss.
|Green Moss Agate (moss included chalcedony)|
An Aventurine is a chalcedony with lots of sparkly inclusions.
|Raspberry Aventurine (red included chalcedony)|
There are many, many, many other misnomers, but I'll get to those in later posts.
If you ever see anything defined as a microcrystalline quartz, you will know that it is a chalcedony. The translucency, inclusions/impurities and banding or lack of will tell you what kind.
(I have no clue why the images are so gray. They weren't before I uploaded them. Very maddening!)