An important omission from my earlier post on polishing this
stuff is the reaction of the stone to heat. There is a quite
dramatic colour change effect. This takes place at fairly low
temperatures. It seems to start at about normal dopping
temperature as a light brown then changes to a quite brilliant
yellow colour, returning to the original colour upon cooling.
An alcohol lamp or cigarette lighter make for a quick cheap
tester as this seems to be a characteristic. Any soot easily