Alabaster is the massive variety of the mineral Gypsum which is calcium sulphate. There is only one true alabaster but there are innumerable different names given to it due to the many different colours (due to other minerals such as iron oxide which...
Call an antique dealer or the show Antique Road show (they have a website) has antique dealers & they may be in your area or able to recommend a great resource for you.
i've bought two chandeliers from pottery barn in the past few months, and they haven't caused any problems yet. so i say go for it. if anything happens to it, just return it.