Indestructible Blocks are blocks that cannot be destroyed by tools (rockets) and devices (dynamite) other than the delete tool.
Currently, there are 4 indestructible blocks. One being bedrock, the other three as materials similar to ore found deep underground in real life.
Bedrock from the underground cannot be broken by the delete tool either since it limits the digging path by going straight down.
One of the others is mostly in the Alien World that also has blocks that have sand on top of them but still cannot be placed but destroyed with the delete tool still.