Tips for cabbing Brazilian Iris Agate? UPDATE, new pics

Sounds intimidating, think a bigger cab or a smaller one will be easiest? 

I agree: bigger is better if the stone has iris all across it or a nice overall face-up pattern. Depends on the agate, though. Lots of agate with iris has dead areas, so you're going to decide based upon the individual piece. I'd suggest also that you design your cab more to go with the banding pattern (which will show most of the time, when it isn't backlit). If that means you cab a smaller stone(s), then that's ok, too.

Smaller cabs can sometimes be as interesting as big pieces (this one is 3mm thick, which is about as thin as I'd suggest before just doing it flat):

That's Indonesian stuff. The light peach base color does tone down the rainbow colors a bit more than in white/colorless agate. Brighter in person than my photo, though. Iris is so directional (worse than opal though not nearly as bad as fire obsidian IMO), and I'm not anything like patient enough to take 50 or more photos in the hope that one of them will hit the angle of maximum iris effect. Having 2 eyes looking at slightly different angles also means that what we see is richer than the single angle cameras see. Thought the cab might hint at the idea of taking into consideration the banding pattern itself, apart from the iris.

Personally, I think the colorless agate best shows the iris phenomenon. Used to have quite a few framed and masked, but am down to one or two sitting around. Colorless Crooked River limb cast:

Great question. Unless you know that a locale has produced iris before, then you're probably not going to find it in just any agate from elsewhere - possible, and you could experiment with something that looks like it might do something by cutting a thin piece and see whether it has those micro-fine bands, however unlikely. The bands that filter the light to produce the rainbows are too fine to see by the unaided eye. They do appear fuzzy, sometimes even milky, with a bit of shimmer that clues you that something could be worth slabbing off a thinner piece.

Maybe one more photo would help. Here is the same Crooked River slab lit from the front (the upper part of the first photo above or lower part of the second photo; the pinkish color is just my hand underneath). The fuzzy, whitish ribbons contain the microscopic bands that produce the Iris phenomenon:

Wow those are beautiful.  I got lucky with the "how do you know", the seller told me when I bought the lot!  I totally forgot it's going to be difficult to check against a backlight as the dop stick and wax will be in the way.  I'll just thin it down and see what happens I guess.  Starting with the one with the black band.

Ok, so I don't see any rainbows but it does look like it's kind of firey?  Is this what it's supposed to do?

3.3mm at the apex, can't go any thinner.  I'll work on the back see if I can hand polish the scratches out.

Can you "screw" up if you dont get the direction right when working them?

I ask because I know with obsidian it happens (at least with me)

More a matter of not cutting the slabs at the right angle. If the bands are oriented so they are not approximately at right angles to the slab, then any iris effect tends to get deflected and lost into the interior horizontally (just as it gets lost if the slab is too thick). If the slab was cut correctly for iris, assuming iris micro-bands are in there, then a low dome shouldn't much affect the display (and flat/tablet top shouldn't affect it at all).

There should be some indication of iris on a raw slab (by wetting it and holding it up to a light source). That's how you'd choose an area around which to design your cab. If there is not even a faint iris effect, I'd wonder how a seller would know in order to advertise it as "Iris"?

The microscopically fine bands actually are fine enough to block certain light wavelengths and let others pass through, depending on the angle. They act as a grating (sort of like polarized filters block out certain light waves). If the angle of the slab cut is wrong, then the light waves hit the boundary of another band, then bounce around (producing translucency), instead of some colors/wavelengths passing straight through.

Brazilian and some other Iris agates are cut from nodules, where the bands follow the curve of the roughly spherical nodule. In such material that does have iris, it is often only the centermost slabs that show the effect. As you slice further toward the ends of one of those nodules, the angle of the banding rapidly curves and the light going through those bands would be blocked or wrong to let light waves through. Maybe an illustration might help understand the difference between the bands in a center cut, and the blocking that gets progressively worse as successive slabs get cut away from the center...

Sorry for the blurry lines, just a quick drawing. There are probably better illustrations out there that explain the iris phenomenon.

Perhaps on some of it might be uncovered if the dome angle was just right, but more likely not. If this cab isn't showing iris colors at any angle, then perhaps flattening off the dome on a lap might find it. It is an attractive cab as-is, so perhaps not worth the risk.

You've explained it perfectly 

Thank you!