Board Thread:Speculation House/@comment-29781768-20180104044655/@comment-4010415-20180104051418

It's possible that they look lava-like because she's using lava. I mean, they do seem to be black glass, and lava makes a black glass called obsidian when it cools a certain way. Her sword even shattered into pieces of black glass when she got ready to fight Jaune seriously. Back in Volume 2, Ruby said she fought with glass, and her weapons were a shiny black material, so she was probably using obsidian even back then. It fits with her glass motif, due to alluding to Cinderella.

Ruby said glass doesn't seem to be her Semblance, but it could be something else that still connects with her glass weapons.