I'm surprised that you're getting a green response from a purple/amethyst piece - can assume only that it's caused by some manganese in the glass - can't imagine it's from a uranium content.
Manganese is used in clear glass as a decolouriser, and generally it gives a greyish green sort of response under u.v. - not the bright green from uranium - maybe it's also added to the batch in some coloured glass, but I'm only guessing, and am sure Christine will put me right.

glad to hear you have a u.v. torch - hours of fun will now ensue.
