Polycarbonate is a strong material that can be very clear. I believe that PC replaced CA for transparent parts at the same time that ABS replaced CA for opaque parts in 1963. The vast majority of transparent parts in LEGO’s history are made of PC — but around 2018, LEGO switched to MABS (possibly because PC uses BPA, which is increasingly regulated by the EU), so PC is now only used for opaque parts like hinges and joints. Transparent PC tends to bond to itself when connected tightly, which is why it’s so difficult to remove a transparent cone from a 4L bar, and PC minifigures only exist as rare delicate prototypes. The PC LEGO uses is Lanxess’s Makrolon.
[Original post and comments on Flickr.]