Obsidian is a glassy rock produced as a consequence of rhyolitic volcanic eruptions. It was highly prized during the Stone Age when it was found to produce good sharp edges, suitable for tools and weapons, when fractured. Michael Grant remarked^[661] ‘”it is the first traded substance of which there are material remains”.

Robert Ishoy who advocates a Sardinian location for Atlantis suggested^(a) that obsidian, “commonly used on ancient Sardinia” was in fact the mysterious orichalcum referred to by Plato. This is quite improbable, as obsidian would not easily lend itself to being used as a wall cladding.  Ishoy’s idea is even more impractical than Jürgen Spanuth’s proposal that orichalcum was a reference to amber. Apart from that orichalcum was described by Plato (Critias 116b-d) as a metal not rock.

^(a) http://www.atlantisdiscovered.org/thesis.htm