Thanks all for your input.
I discovered that I had another one of these that I had turned round earlier.
Yea, I know, a sanded shot of the flat would have been helpful too.
Anyway, I took the other one and did a quickie CA finish and polish to see if the effect remained. It did! I took 2 shot but turned the blank just slightly as you can see by the reference mark at the top. Most of the darker brown seems to disappear.
It has no odor of any kind and it is quite hard so camphor and poplar are out. Actually it seems harder than most of the maple that I have worked with up to now. I have a pen made from figured maple that has a little of the chatoyance effect but not nearly as much as this one has.