Favorite fortifications - agates that is

kw: rocks, minerals, agates, chalcedony

I was going through some rocks recently, getting a load ready for my tumbler. I've been mostly tumbling jasper that I got at Lavic, California (I posted about Lavic Siding and its wonderful Jasper in March, 2008). But I also have a bit of this and that, different stones of different types, that I've acquired over the years. One thing I am very short of is fortification agate. I made up my mind to acquire some. There is a rock show in Delaware in March, where I'll look for pieces to buy. I'd love to get a piece or two like this one:


This is a Fortification Agate. This sort of pattern is the classic Agate. Some old Aggie marbles were carved from well-formed agates and show patterns like this. This image is a clip, a bit color-brightened, from a photo at Superagates, of a Fairburn Agate, the original Agate. All other fortification agates are compared to Fairburns, with good reason. They're the best.

There are many kinds of fortification agates, for they occur all over the world. Some have larger, coarser patterns, and some are smaller and clearer (the rock above is about two inches, or 5 cm, across). Some are more or less distinct or colorful. I do have a couple of rather insipid agates from the Laguna locality, but I'd hesitate to put up their images; almost a waste of the space.

Agates are nearly pure quartz; the impurities are mineral salts that provide the color. Nearly every color is possible, but earth tones and red-orange are most common. The quartz in an agate is crystalline, not glassy, even though they look like stained glass. They are cryptocrystalline, which just means that the crystals are so small they are hidden from our eyes. The two main types of cryptocrystalline quartz are flint and chalcedony.

In flint, the crystals are blocky and don't have any particular orientation. In chalcedony, the crystals are filamentous, like short fibers. In some forms, the fibers are not oriented in any particular way; they are like felt. Most jasper is composed of felted fibers. When the fibers are oriented, we get agates.

In fortification agates in particular, the fibrous crystals are oriented radially; the bands are formed of crystals that run the short way. In some cases, the crystals are very transparent and when they are just the right range of size, they diffract light. Then thin (2-3 mm), polished slices of the agate show iridescence, and this special kind of fortification agate is called Iris Agate. An example is shown here, in an image from the Flickr account of Rockman836:


The colors don't come from pigments, they come from the light diffracted from the oriented fibers, and the color shift results from the changing orientation of the fibers with respect to the light source. On my monitor this image is very nearly life size, taking for scale the binder clip holding the agate piece. That's quite a large chunk of iris agate. Most of what I have seen has been smaller (of course, I may be mistaking the size of binder clip that was used).

By the way, one would never tumble an iris agate. They need to be cut thin and polished on a flat lap. Other fortification agates can be tumbled, and I have seen rather large agates that were tumbled so their patterns show from all surfaces. Awesome!

Need you ask why I don't go collect my own agates? Compared to jasper, good fortification agate is quite scarce. Many of the classic collecting localities are nearly worked out, and some have become private preserves. If you look at the prices on e-Bay, you'll find that agate isn't really a "semi-precious" stone any more. At current prices, it is as costly as Jade. But buying some will cost me less than a collecting trip, so I'll celebrate oncoming old age by "silver picking" my next acquisitions.