[swarmee]

Yes, for the most part, these are authentic game-used jerseys and bats bought from reputable auction houses that are being cut into tiny pieces in order to be put on cards. Topps, Leaf, Panini, etc would be able to tell you exactly what piece they used for the manufacture of those cards.

Some of the memorabilia bought by those companies have been proven to be fraudulent, but that's mainly because they were originally authenticated improperly or sold by auctionhouses that recanted their originality. Topps is normally on the hook at that point to remake the card with authentic memorabilia. They have had to rebuild cards with Babe Ruth cut autographs when some were shown to be forgeries.