tolkiens legendarium - What Element of the One Ring Could the Nazgûl Smell?

In The Fellowship of the Ring Frodo's first experience with the Nazgûl is described.

When [the Nazgûl] reached the tree and was level with Frodo the horse stopped. The riding figure sat quite still with its head bowed, as if listening. From inside the hood came a noise as of someone sniffing to catch an elusive scent; the head turned from side to side of the road.

The Fellowship of the Ring - chapter 3 - Three Is Company

In the Jackson movies the Nazgûl also try to ferret out the ring by sniffing and smelling for it, first in the scene where Frodo, Sam, Merry, and Pippen encounter a Nazgûl on the road after they fall down the hill from Farmer Maggot's fields, and then, when Frodo and Sam and Gollum are crossing the Dead Marshes, a Nazgûl flies overhead on a fell beast sniffing for the ring.

So my question is what element of the One Ring could the Nazgûl actually smell? Is it the metal the ring is forged from? Could Sauron's evil and malice and hatred have an actual scent, even if only to the Nazgûl? Why do the Nazgûl sniff and smell for the One Ring? I'd prefer a canon-based answer if possible, especially from any of Tolkien's books including the supplemental volumes.

Answer

A quote from Aragorn, The Fellowship of the Ring, A Knife in the Dark:

For the black horses can see, and the Riders can use men and other creatures as spies, as we found at Bree. They themselves do not see the world of light as we do, but our shapes cast shadows in their minds, which only the noon sun destroys, and in the dark they perceive many signs and forms that are hidden from us: then they are most to be feared. And at all times they smell the blood of living things, desiring and hating it. Senses, too, there are other than sight or smell. We can feel their presence - it troubled our hearts, as soon as we came here and before we saw them; they feel ours more keenly. Also, the Ring draws them.

From The History of Middle Earth, The Return of the Shadow, "Of Gollum and the Ring," describing what happens when one is fully possessed of the Ring:

Yes, if the Ring overcomes you, you yourself become permanently invisible - and it is a horrible cold feeling. Everything becomes very faint like grey ghost pictures against the black background in which you live; but you can smell more clearly than you can hear or see. You have no power however like a Ring of making other things invisible: you are a ringwraith. You can wear clothes. (you are just a ringwraith; and your clothes are visible, unless the Lord lends you a ring) But you are under the command of the Lord of the Rings.

The indication from Tolkien's writing would seem to be that the Ringwraiths simply use their smell like we would use sight and sound. They can smell living things, but there is no indication that they smell the Ring. They can sense its presence, however.