There are three basic types of glass colours - opals, transparents and opalines. What is often confusing is the difference between opals and opalines, because when an opal is blown thin it can appear to be opaline. As an artist I have never got too deep into the science part of this, which I know fascinates a lot of members, but it is to do with how the molecules align themselves as the glass cools and the difference between colloidal and non colloidal colours. I am sure that Adam D could enlighten us much more than I can!
Anyhow, although it hasn't been codified, there is a convention that is used by most colour manufacturers, and therefore many glass makers, that transparent colours are given the names of jewels and gem-stones...I think this dates back to the 18th century when there were very few colours available, and the earlier colours were sapphire (Bristol Blue), emerald, amethyst and in the nineteenth century ruby (Cranberry) Opals and opalines came later and "purple" is definitely an opal colour!
Forgive me if my chronology is out because this is all from memory. To confuse things even further, there is a difference between European and English terminology - I mentioned Bristol Blue, which some people erroneously refer to as cobalt in the UK, because they are more familiar with ceramics! I don't think this was originally referred to as sapphire in the UK but I'm pretty sure that in the middle of Europe, and also at Baccarat, that they called it sapphire.
I'm probably going to get into trouble here, but many of the great colour alchemists have been from what used to be called Bohemia and the main colour manufacturers are still in that region. In recent years, these are Kugler (Freidrich and Schreibler), Reichenbach (effectively Klaus Kugler's second factory), Zimmerman, Ornela, (now part of the Jablonex group but formerly Wiesenthalhutte and then Schott). The recent emergence of Gaffer in New Zealand has begun to challenge the market and also the terminology by effectively negating much of what I've said above!
Anyway, if you asked me for a sample of amethyst, it would definitely be transparent, and if you wanted purple, it would be opal. Different tones of amethyst have different names.. for example blue amethyst or reddish amethyst (amethyst blaulich and amethyst rotlich), aubergine, hyacinth but not purple!