When using Stone Skin, whether inherent or spell-based, your skin really is turning to stone. This is demonstrated by the fact that you can cast Stoneshatter on someone with stone skin, and deal ===OBLITERATES=== damage with an accompanying echo, "Bob screams in agony as he skin shatters!"

Therefore, the echo, "Your skin turns to stone," is the accurate message. If anything, the help file should be updated to accurately describe the effect as turning your skin into real stone.