The hardness of cobaltite is 5.5, which is similar to a knife blade. Silver would be scratched by a knife blade and be malleable (i.e. doesn't crumble when scratched).

What is the hardness of arsenopyrite?

Arsenopyrite is an iron arsenic sulfide (FeAsS). It is a hard (Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elemental sulfur.

What is cobaltite used for?

Cobaltite is a rare sulfide mineral, used primarily as an ore for cobalt.