Crystalline or vaporous element (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
iodine
(Other definitions for iodine that I've seen before include "Antiseptic found in seaweed" , "Element that can exist as a purple vapour" , "for therapeutic purposes" , "Black/violet element" , "Mild antiseptic" .)