The hardness of minerals or substances is measured qualitatively using Mohs Scale. In this scale the hardness is measured qualitatively by characterizing the resistance shown by one substance when rubbed against second substance. Therefore, greater points are given to those substances which are capable of making scratches on other substances. The Mohs Scale of given compounds is as follow,
Diamond <em> 10</em>
Fluorite <em> 4</em>
Topaz <em>8</em>
Gypsum <em>1.5 to 2</em>
While the hardness value of Fingernail is <em>2 to 2.5</em>. Hence, this value is close to Mohs scale hardness value of Gypsum therefore, gypsum is the correct answer.
