Watermarks can significantly increase the value of a stamp. For example, Canada stamp #28 in Mint VF (Unitrade listing) has a catalogue value of $1,400, but the same stamp with a watermark (#28a), is listed at $6,000! Obviously, it’s a good thing to find a stamp with a watermark. It can mean the difference between an ordinary stamp and a rarity. So then, what is a stamp watermark?