The Collaborative International Dictionary of English v.0.48:

Sclerometer \Scle*rom"e*ter\, n. [Gr. sklhro`s hard + -meter.]
   An instrument for determining with accuracy the degree of
   hardness of a mineral.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sclerometer
    n 1: a measuring instrument that measures the hardness of
         materials by penetrating them with a stylus that has a
         diamond point