My prototype molds are made from 6061 aircraft-grade aluminum. It is a relatively soft aluminum, and machines easily. This makes it ideal to make small injection molds. I use them to produce tests of my designs, and when I was first cutting them (oh those many years ago), I had a small, hobby-grade CNC which was difficult to keep tuned, and was limited to cutting at only 10,000 RPM. This meant that for me to produce high quality molds, it took a LONG time - 8 hours per mold half, and sometimes even longer! So, as a compromise, I cut them with reduced resolution, which yields a rougher cavity.
I have since upgraded to a much larger and more robust machine with a very highspeed spindle, as well as a better injection molder, which allows me to cut high quality molds, and inject with a very high success rate. Parts from my newest molds are indistinguishable in quality from my production molds - but each is injected by hand, which is slow. Also, once I inject the plastic into my prototype molds, I have to pluck the part out by hand. Sometimes the part wants to stick, and I have no recourse but to grab a utility knife and pry out the sprue.
These are made from hardened steel, and are produced by professional tool makers. These expensive tools typically weigh over one hundred pounds, and are designed to last up to one million injections before wearing out. They also incorporate pins to push the parts out of the mold after injection, and the tools are designed to operate continuously for days at a time. My first production tool was made in early 2006, and is still in operation today.