The Hexagram of Solomon?
Well... It was never orignally made, let alone from the same time period as we know it from the "King Solomon" of the Israelites...
If I recall it may be found within the writings of the "Greater Keys" or "Lesser Keys" of Solomon...
Though I feel that was later put in as with many other things, as the years went on/as it was rewritten/etc.

Some sckeptics believe it was an ancient tantric symbol of fertility.

I would say it is even older than the vedas, and not just a fertility symbol... But rather the divine union of the mircocosom and macrocosom... (as above... so below) as the say.
But the true insight to that:
(There is no "up" and "down")

Imagine all existence as two mirror type orbs.
one in the middle... one surrounding it.
they reflect off of each other into infinity in every direction possible.

But for practical use of it...
I suggest reading some of Crowleys work.
Or maybe Israel Regardie.