I’m glad someone posted a picture. Those are caused by lightning strikes and you can buy them on etsy or ebay. My BBE studies lightning, so I asked him about Lichenberg figures and if they were real. They seemed fake to me because of how beautiful they are. He said they were absolutely real and they presented one at the retirement of a past Laboratory Chief (now he’s the chief) . You can read about them here: https://en.wikipedia.org/wiki/Lichtenberg_figure
They happen in insulation material, but less dramatic because it isn’t done on purpose under controlled conditions. They used to use glass, but much of what we see now is porcelain. The glass was a strengthen, tempered glass, like Pyrex. when it breaks it shatters because of the pent strength in the glass and makes these types of material less desirable (if you want to learn about the physics of this you can read here: It based on the principal of ‘Prince Ruperts Drop’ https://www.youtube.com/watch?v=xe-f4gokRBs