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Very much like the inter play of light in opals

[attachment=0]Opals Of The World (1).jpg[/attachment]

I didn't know about opalised petrified wood -- I see those those flashes of color in the photo, thanks for posting.

However, one thing you may not see very often, because it's incredibly rare, is an opalised petrified wood. When the wood, a tree for example, lies on the ground long enough, minerals enter the tree, while the wood rots. Eventually, the minerals take on the form of the wood creating petrified wood. There's a whole forest of petrified wood outside San Jose called the petrified forest. However, during the formation of the petrified wood, rarely, bubbles begin to form of just the right spacing. The petrified wood is now also an opal.

Thank you Din, this is amazing.

You're right about labradorite and opals. With opals, air gets into the stones during formation creating a random set of bubbles. The spacing of these bubbles, being random, creates the flashes of colour that change as you move them around. Wit labradorite, there's only one, sometimes two, planes, so thhere are no flashes, but there is Bragg selectivity - just as in a volume hologram. We've been collecting gemstones for about 15 years now, and wife, Joy, is particularly fond of opals because of the connection with holography. Here's an illustration from a book about opals (Notice, they reference C V Raman, the one who discovered the Raman effect):

[attachment=0]opal.jpg[/attachment]

I was wondering what you might pick. Having a new interest in minerals and gems, I was surprised to see some effects which, I believe, are the same thing as a hologram.

With reflection holograms in mind, I would say that holograms are repetitive regular changes in refractive index, where the distances for the changes are around the wavelengths of visible light. Another requirement (for a reflection hologram) would be that both index-changing regions are transparent. (Maybe I'm missing other requirements.)

I have a couple samples of labradorite (one of which is called spectrolite), and have seen some opals in a gem store. All the color effects (and the appearance of the reflected wavefronts of light) seem identical to a reflection hologram. The Australian opals are amazing, the intensity of colors seems to surpass any hologram I've seen.

[attachment=1]spectrolite.jpg[/attachment]

[attachment=0]opal.jpg[/attachment]