I like the idea of making pots using lava rocks. And the grey coloured lava you have. Here are some lava rocks that can be found here. The colour is brown-red, similar to lava granules imported from Italy. It's so soft I can use a screwdriver to make sth out of it. The only thing that keeps me not to use it for making a pot is it's porosity. When watered with tap water it will get stained white from calcium... the same can happen if chemical ferts are used. How will you solve this?