The story
A genericization from a trade name of the 1940s, Perma-Stone, itself a proprialization from what had previously been a lexical gap affixation, perma- + stone.
Etymology from Wiktionary, CC BY-SA 4.0.
permastone
Today it means: Any superficial surface constructed to look like stonework, usually made of concrete or thin sheets of rock assembled over an existing surface. Full definition →