Well, it may be that the deep deep blue that is common in opal is what they consider violet. I've always considered it blue. lol.
Looking at the wiki page for Violet, when it gets to discussing the wavelengths, there is a "blue" that is officially in the "violet" range that I think qualifies for some of the color seen in opal, but it's not a huge deal. I wonder if purplish violet tones don't show up as much because the size gets *too* small, and the structures have a harder time lining up? I'd be curious to see what the people who study opal on that level think about that. You really don't see purple that often, as I recall.
Purple and violet in opal are quite rare in my experience. I can't explain it from a technical standpoint but I have what may be a little insight into how the colors are named. First, violet is a spectral color if the old mnemonic I was taught is still right: V-violet, I-indigo, B-blue, G-green, Y-yellow, O-orange and R-red -- VIBGYOR.
But color systems used to describe gem colors vary a lot in terms of names. My old stand-by is the Gemdialogue system but there are several others, Munsell, the now obsolete GIA gemset by Pantone and the new one they've adopted, Gem-e-Wizard along with others. Violet in the Gemdialogue system would be called "B2P/moderate bluish PURPLE."
The best showing of violet in opal I've personally cut is in this crystal opal from Mintabie, Oz. It's hard to capture the exact hue in an image but depending on viewing angle the entire top third of the stone shows a stunning pure violet.