Of the more than 1,600 mineral forms, only about 16 excel because of beauty, color, hardness, brilliance and rarity. Meeting these qualifications, they are called gems. The harder ones are rated as precious stones, and those below the number 8 on the hardness scale are considered semiprecious.

“Hardness is based on Mohs’ scale of hardness. The harder a gem the more durable and valuable it is, as it can be given a higher polish. All the higher-grade minerals can scratch those of a lower grade when rubbed together. The highest number is 10, for the diamond, while the quartz family registers 7. In comparison, a steel knife rates 6.5, and the hardness of your fingernail is about 2.