In the list you have ten substances. Kaolin, the base, is 1, Rock
Salt is 2, which indicates that Rock Salt is twice as hard as Kaolin.
Iridium is 6, and six times as hard as Kaolin. Diamond is 10, which
means that it is ten times as hard as Kaolin.
Learning this list by picturing reminders with the word of the Code
list will enable you to easily recall these ten substances and the
degree of hardness compared with Kaolin.