Why clear glass shows green under uv?

i thought uranium glass was only pale green, yet with a uv torch , it glows green, yet other glass i have doesn't. and does this make it old?, or is it recycled glass?Why clear glass shows green under uv?
It's called ';Vaseline Glass'; or ';Uranium Glass'; and was common during the depression. Here's a reference: http://www.vaselineglass.org/