Answer:
Tom Brocher emphasizes that the strand of the San Andreas Fault in the area of Hayward, CA. is creeping.
What you're looking for in here is the hardness: a material can scratch another material if it's harder than it.
So the desired material cannot be harder than amphibole, which has a hardness of 6.
But it has to be harder than fluorite, which has a hardness of 4.
So we're looking for a hardness of between 4 and 6 and from the table we see it's magnesite (we'll need a hard magesite) - this is the correct answer!
