A while ago, we looked into a seller having transparent minifigures for sale - sooner or later we knew we'd see one of this. A full transparent Darth Vader LEGO minifigure for sale on eBay. If there's any one figure that I'd sell a kidney for, it'd probably be this.
I stumbled across this listing and it's currently (as of Sept 10 2019) going for USD 1,299.00 for a Buy It Now by owned by seller ting.l8. You can find the listing here while it lasts.
The minifigure is real if you've got the dough to blow.
Image Source: eBay seller ting.l8
In case there's any doubt that the Darth Vader helmet or any other part is not a LEGO element, this hopefully will take your doubts away.
Image Source: eBay seller ting.l8
My favourite image is this photo that's taken right in front of LEGO's offices in Billund.
Image Source: eBay seller ting.l8
We've since learnt from AFOL Edd Pearson of the following about transparent polycarbonate minifigures in general...
"...they aren't made of ABS like regular minifigs.. These are polycarbonate, which is much more brittle than ABS. This is the reason they don't make them "officially". ABS doesn't stay clear once molded, so they use polycarbonate. Polycarbonate parts also have a much tighter clutch power, which causes more stress on parts and can make them crack. LEGO have strict rules about which parts can be made with polycarbonate too. They do make these, for internal use, I've seen many of them in LEGO offices"
The next time you're in LEGO offices perhaps, see if you can ask a friendly LEGO employee to spare you a transparent minifigure or two!