My first exposure to crochet was watching my mom crochet covers to protect our couch and chair arms from the kind of wear and tear that happens. She used heavy thread, I think #3, and did her own pattern of chains and doubles to create a version of what is called 'filet crochet'. That's when a pattern includes closed and open spaces and, if carefully planned and charted, it's possible to create simple pictures and text that way. Think "Victorian sampler." Usually that's done in a much finer thread.
Thread is rated according to a number system that goes up as the thread gets finer. A #3 is roughly equivalent to string. It's very sturdy and often used to create things like dishtowels and facecloths because it will wash and wear very well. #10 is what you usually see used for doilies and such. #30 is extremely fine; more like sewing thread. And I think it's possible to get thread even finer than that, although you probably can't find it at your local hobby store. If you are looking for fancy lace trim on your wedding or ball gown, that's the stuff you want. The finer it is, the more drape the fabric will have. Makes sense, right?
Mostly it's made of cotton. In recent years, a blend of cotton and bamboo has become trendy. Bamboo is incredibly soft, but also terribly stretchy with no elasticity, so blending it with something else is a good idea. But it's lovely stuff for wearables. 100% cotton thread tends to feel a bit stiff when you are first working with it; but it loosens up considerably as it is worked and more so with blocking, the way an old favorite t-shirt will soften over time. I've also heard of silk thread, although I've never seen it in a store.
When I was a kid, people used doilies and lace tablecloths a lot to protect a finish and show off their whatnots. Today, you don't see it so much. I think the current trend is for less clutter and to show off the beauty of wood furniture. We've gotten away from using doilies, table scarves and so forth. I think, though, that it's going to come back around again eventually. Most things do.
The above projects, in case you were wondering, were done with #10 thread. I did the bookmark first for a small project. It was a way of getting my feet wet with thread. The second is a beaded scarf, still in progress. The beads are about the same color as the thread, but I think you can just make out their glint at the edges and at the motif centers. In the third photo, I've included the two crochet hooks I'm using. One to work the thread, and one to work the beads. Yes, there are hooks on the ends of them. One hook is 2.25 mm, and the other is 1.3 mm. The pen is by way of comparison.
These tiny thread hooks are made of steel. In theory, you can make a crochet hook out of nearly anything that's rigid. Most hooks used for yarn work are aluminum. I have a set of bamboo hooks, too. But thread hooks are inevitably made from steel because the heads are so very fine, and yet need to stay rigid. Aluminum hooks that tiny would bend. Other materials, if you could carve them to such thinness, would be fragile and probably break.
And that's your lesson on thread crochet.
Here's something else to look at. It's a baby afghan I'm almost finished with. I'm using yarn, not thread, so I can use my bamboo hooks. In this case, it's 6.5 mm. I'm using the Tunisian stitch in a technique called entrelac, which makes it look like woven strips to get the diamond pattern. I think I talked about Tunisian before. It's a cross between crochet and knitting, neither one nor the other but incorporating elements of both. You use a hook - that's the crochet element. But you cast on a series of loops, which is more like knitting.
And that's your refresher on Tunisian.