The first seal, found at Harappa before 1872. Included in The British Museum's A History of the World in 100 Objects, a nice podcast of the chapter on this black stone unicorn seal is available for free at bbc.co.uk (Episode 16, Indus seal). Sir Alexander Cunningham, who led the first excavations there in 1872-73 and published news of the seal, wrote 50 years before we understood that the Indus civilization had existed: "The most curious object discovered at Harappa is a seal, ... The seal is a smooth black stone without polish. On it is engraved very deeply a bull, without a hump, looking to the right, with two stars under the neck. Above the bull there is an inscription in six characters, which are quite unknown to me. They are certainly not Indian letters; and as the bull which accompanies them is without a hump, I conclude that the seal is foreign to India." How wrong Cunningham was about the seal! Still, as Neil MacGregor writes, "it was this seal that stimulated the discovery of the entire Indus civilization." (A History of the World in 100 Objects, p. 80). It was left to a successor, John Marshall, to announce the discovery of the ancient Indus civilization.
Thanks to The British Museum for the image.