So.. My internet was off and I decided play around with the blackbody node. IT IS AMAZING!

Here is what I came up with in the hour my internet was off.

Warning these images are lossless and quite big it may take a minute to load. Also if you click any of the renders are the top you will leave this blog post. I recommend clicking it with the scroll wheel aka middle click it will open the images in a new tab.

Now I am going to show you how to do the lava part which will look much better if it isn't on a cube ;)

Here is the node layout without the crazy mapping nodes

So you will first need to know what the black body node does. Basically it works off of heat the Cycles node uses kelvin. Here is a graph that explains it with the minimum amount of nodes you can have to use it

the multiply node is to boost the levels from the noise texture to 4000 so they are orange in the blackbody node.

Notice the image isnt black and white aka 1 - 0 its more of a blend between 2 greys.

Black and white should work better and have more accurate color levels based on your input temperature (The multiply node) but would have a sharper falloff.

Here you can see the out put from the color ramps mostly for masking.

This one converts the noise to a black and white image (Not grey to grey like shown above)

This one defines the area that emits light (The white parts)

Here is a more advanced version of the nodes which just makes the lava swirl when its animated

Here is the blend if you want to play with it. It is CC0 so you can do what you want with it.
http://theorysend.com/view/38699e6579a7396e97db3441a921c27255ad2c43

The first few mapping nodes in the advanced image are almost impossible for me to explain here. There is a 1 hour video dedicated to explaining it on the BlenderFoundations YouTube channel it is a must watch if you love to make procedural things in cycles.

Here are the nodes from the mushroom that weren't supposed to be seen by anyone else lol

Here are some videos including that one on I was talking about on the BlenderFoundation page