I've never been a big collector, not since I was eleven and had a herd of ceramic horses, and I've never been inordinately obsessed with buttons. But now my work area overfloweth and I have to admit they're pretty cool.
I think my favorite is celluloid. This was one of the earliest plastics, later abandoned because it was found to be unstable. Millions of buttons from the 30s and 40s are available today, in all shapes, colors, and sizes. Here's a picture of some of my current beauties:
These celluloid buttons include bubbletops (on the top row), a tight top (far left), and three wafer buttons. The far right one is a layered and carved wafer, one of my favorites types. The bright blue one is called a buffed celluloid (not sure if you can tell in this photo). Bubble tops and tight tops all have metal backings; the difference is in how close the celluloid is to the metal.
Now here's what some of them look like when they're made into jewelry:
On the left is a bubble top, paired with a chunky lucite button (a later plastic). You can really see the depth on this one. To the right of that is a layered wafer, carved to reveal the different colors underneath (the button on top is, I believe, a Victorian cut steel). Finally, below is a pressed wafer button (this one is topped by a carved vegetable ivory button and mother of pearl).