Rayskin has some mythical aura around it: especially about the question: what is it?
Main problem is the use of the term (and with this: the kind of leather meant) during time. Used in the beginning for embossed horseskin, it now applies to fish leather.
I think many people can say how it looks, but not how it is made. I will try to reduce to 'problem' to its sources and rework history from there.
At the moment the horseskin variety is most interesting to me, and the reported use of Chenopodium seeds to make the whirling dense patterns in the leather, and the tradition of green dyeing for these leather. At least, besides the Chenopodium seeds as printing device, there must have been tools for making these patterns. And not even exclusively used for horseskin: a lot of the 'chagrin' leather is simply printed goat or sheep leather. Not that there is anything wrong with that, but for the terminology, confusion is complete in this case!
A conservator at the Dutch Army Museum, Mathieu Willemsen, wrote a nice article on the use of rayskin and its leather pendant, which can be found here
... to be continued...