The Strange Rock artifact can be found at Udûn Foothills, in the central western part of Udûn.
Strange Rock (volcanic) Second Age, circa 1600
"Orodruin's eruptions are sudden and violent. When the mountain awakes in fire, none are safe. The skies darken with smoke and ash, and fiery rocks are shot high into the air before landing with deadly force.
This glassy, obsidian stone is one of those volcanic rocks; it dates back to the Second Age, perhaps to when the Dark Lord formed his initial bond with the volcano. These remnants of Orodruin's fury are prized for their hardness. They are often sharpened to make weapons."
"Where'd you get off to, child?
I found this, daddy.
This kind of rock is very rare. It comes from the volcano. There!
Now, Doom is a foul place, angry and mean. It will explode when Sauron is near and plotting, spitting all manner of foulness into the air: smoke and dust and rocks that burn the ground. It does this as a warning: I will not stay silent for long!
Sort of like your mother.
Don't tell your mother I said that.
So this rock is…
It is a rock, nothing more. And you needn't be afraid of a rock. Or Doom, which is silent to this very day.
And, sweetie, if you need a lesson from all this, why, here's a fine one: Even a foul place like that volcano can produce beautiful things."