Silk is, without question, one of my very favorite fibers. Nothing quite says "luxury" like a little silk Which is funny, given that silk is basically caterpillar spit. It is made from the cocoons of the silk worm, a little caterpillar that eats mulberry leaves. It is not a vegan fiber. The first step in the process of turning cocoons into yarn is dropping them into BOILING WATER! Needless to say, the little larva isn't coming out of this one alive. After that, either an end is picked up and the cocoon is unreeled from there (reeled silk), or the cocoon is stretched out over a frame. If you're a spinner you may recognize this part -- it's where we get silk hankies, caps, and bells.
The biggest misunderstanding about silk, I think, is that everyone thinks of it as an extremely fragile fiber, handle with care. This is both true and false. Individual silk fibers are actually incredibly strong, as long as they are dry. They are just incredibly fine, so they catch on things easily, and shockingly fragile when wet... especially if it's hot water. That's why they dunk the cocoons in boiling water before they unwind it. It opens up the fibers and softens them.
What does this mean for us silk users? Well, silk does have to be washed very carefully, lest it break. I treat silk like I do fine wool that I'm afraid will felt. No agitation, no wringing! The other option is to get it dry cleaned. Once it dries (flat!), however, silk is pretty durable. I think that silk blends are great, for example, for a very special baby item that you wouldn't mind hand washing. My youngest's coming home sweater, for example, is a silk blend.
Pure silk is also very expensive, generally speaking. There are some exceptions. If you were to say order your silk from eBay, where there are a number of sellers from overseas who carry 100% silk yarns, you can get a real steal. That being said, it's difficult to say if it's really silk, or if it's actually rayon. Of course if you conclusively proved it to be rayon, eBay's fraud department would help you out, but it's difficult to prove one way or the other. That being said, there are a number of very affordable silk blends. I'm very fond of Misti Alpaca Pima Cotton and Silk, and of Crystal Palace Yarns Panda silk. Both give you that sheen and silky hand, without the crazy price tag.