Seal ink is nothing like ink for fountain pens or fine liners. It is a thick paste made up of cinnabar (or mercuric sulphide), castor oil and aged mugwort all pulverised and mixed together. Mugwort is a plant.
And you leave a lovely red imprint. When I sign books and use the seal, I always end up with the red ink all over my hands and often on the table cloth, if who ever is hosting me has been kind enough to provide one.