Probably a copy. but its California gold http://amhistory.si.edu/coins/printable/coin.cfm?coincode=4_06A
It is a copy. Doesn't have the right design and lettering to be an original. There are many copies like this that have been sold for $19.95 a piece and contain no gold. But there are some that were struck in actual gold. Here's a real one... overpriced for a problem coin: http://www.ebay.com/itm/231983647124?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT
By the way. The real $50 slug is my dream coin. Has been for decades. I see them fairly regularly at shows. Anywhere from F15 to UNC details. I honestly wouldn't care about the grade. I just want to hold the giant hunk of California gold.