Sturdy plastic which can be filled either with water or sand.  Just be careful, I regretfully did not read instructions, just took cap out of plastic bag when unpacking (missed reading little bag) and screwed the cap on, so I could transport to fill and not misplace the cap.  Please, DO NOT PLACE CAP ON UNTIL YOU FILL FIRST.  If you accidentally, like I did, place the cap on, I contacted manufacturer for a replacement in case I damaged the cap open, but was told to use a pair of pliers and that would work.  The thread has very good thread and locking teeth (gear like). 

I was smart though, I did not do the same with the other three bases I bought!  hahahahaha!