I created a lava sword for my son as a cosplay prop. Here is the result so far. The hilt still needs work, but I'm very proud of the blade. See below for photos of the process.

I made the sword out of an old bed slat.

First, I cut pieces of wood from either side at the bottom of the slat to make the grip and then I wrapped that section in strips of leather from an old belt. 

I then cut off a shorter piece from another bed slat to make the hilt and cut out a section so that the hilt would fit onto the sword. 

I spray-painted the whole thing with a black base coat and then used wood repair putty to create a rough, clumpy surface, which when hardened would hopefully look similar to lava rock. After the putty hardened the first time, I gave the blade a paint job and then decided to build up the putty a bit more around the lava streams so that the rock would overhang a bit and not seem so artificial.

Finally, I painted the blade again. I used a grey acrylic paint to drybrush the stone and added orange and yellow highlights to the lava to make it appear more "alive". Here you can see a close up of the blade before I did another drybrush of the rock. You can see the final product at the start of this post.