It does look like silk to me – or a silk blend. If I had to guess I’d say it’s Lorna’s Laces Lion & Lamb (or another yarn based on Henry’s Attic Andromeda, a 50/50 silk/merino blend).
They have at least a couple different banners. Click the link to go to Knitpicks yarn list and maybe you’ll see which it is. There was a thread about it last summer, I think, so maybe you could search for that on the knittyboard.
My bet is that it is Art Yarns Regal Silk but I can’t figure out the shade. Keep in mind the color may have been altered when they designed the banner.