These things are not silk yarn. You caught us again.
In fact, as much as they look like something that was delivered direct from a silkworm, they're not. They're by-products of industrial silk-reeling. When the silk is reeled onto spindles, this is the part at the centre that is so tightly compacted and full of sericin (a silkworm's glue) that they can't unreel it. So they cut it off and create one of the world's exceptionally lovely things, silk casings. Some are smooth and some are ridged. Artistic people make them into jewelry; fibre artists embellish things with them. Aesthetes put them in lovely bowls to simply admire.