Diamond is a gem because it is relatively and
a. rare; clear
b. hard; clear
c. rare; valuable
d. clear; valuable

Option (C)

Explanation:

Diamond is the hardest of all the minerals. According to Mohr's hardness scale, it has a hardness of 10. The carbon atoms in diamond are shared by a strong covalent bond. They are formed in the mantle at a depth of about 150 km and beyond that, where the temperature and pressure condition is extremely high. These diamonds are carried to the near-surface region with the help of kimberlite pipes due to the deep volcanic eruption that is initiated. Diamonds are very expensive because they are formed at huge depth and their cost of extraction is really high. They are very rare so they are really valuable.

Thus, diamonds are used as a gem because of their hard and adamantine luster.

Hence, the correct answer is option (C).