The crystals in red arrows mark may cause a burst, some on the rock surface, we making selection and get rid of it, but there are some crystals inside the rock which we can’t see.
The main reason for the burst is inadequate combustion, so we burn it again in the appliance as next picture shows to reduce the risk of it bursting. This is usually seen on black lava rocks, so we don’t recommend customer order black lava rocks.
We offer two colors for these lava rocks, red is our standards, more safe than black, black is customized for our customer which with higher cost.
We also put a warning pater pack with these lava rock to make sure customer use the rocks correctly.