I supplied one which didn't get published. But, let's do it the
other way around. Why don't you supply references which support
your position? I'm sure many of us would be interested.
Chalcedony, in gemological circles, treated with disdain because it
is not expensive, so gemology books do not spent much time on it. But
mineralogical references are quite detailed. I am most familiar with
Fersman, due to my educational background, but I am sure that Dana
will be just as good.
This should be done as a blog, but time is of the essence, so in a
few words as possible. Chalcedony forms crusts, amygdules (gas
cavities), and nodules, act as vein filler, cements Breccias
(sedimentary rocks), and can even form as a pseudomorphs. A lot of
gems like Turquoise, benefit from presence of Chalcedony. Due to all
these different modes of formation, Chalcedony is classified as
fibrous (which in turn sub-divided into parallel, spherulitic, and
feathered), fine-grained, and ultra-micrograined. All this abundance,
in combination with metallic salts present during formation, give
raise to many gemological varieties. For example - if Nickel is
present, the chalcedony is called Chrysophrase.
Agate is defined as variety of Chalcedony, characterized by a
pronounced macroscopic banding. Agates are mineral aggregate, whose
composition includes other minerals, as well as Chalcedony.
Dollinger lists more than 30 minerals, which are included in agates.
This is a vast subject, so it is better for those who are interested
to pick a book or two about agates, then me, trying to cramp
into a few lines.
Since, discussion started from what is the best material for making
a burnisher - simply to say agate is not precise enough to have any
meaning. Burnishing, in it’s own right, can be a subject of many
volumes. From goldsmith point of view, I want to use something which
gives me the best chance of success. Some agates are very good for
this, and some are not. Limiting definition to Chalcedony (variety)
is more precise.