I assume they look brown in transmitted light because of rayleigh scattering from the very fine inclusions. I believe the cat's eye is caused by mineral inclusions rather than growth tubes. I remember someone saying they were sillimanite, though I've found at least one reference to a cat's eye alexandrite with rutile needles: https://www.gia.edu/gems-gemology/winte ... on-pattern, and this 2016 article notes higher levels of titanium in areas with more inclusions, but also suggests that their identity has never actually been firmly nailed down https://www.gia.edu/gems-gemology/sprin ... -australia As to why they'd be zoned that way, though, it'd be hard to say. Perhaps they represent growth zones during periods with less titanium in whatever it was precipitating from, perhaps it has to do with temperature fluctuations favoring exsolution or reabsorption of needles, or maybe something less straightforward.
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