Is diamond the hardest substance on earth and can it ever break?

Diamond is the hardest substance, which means it can only be scratched by another diamond. However, it's not the toughest substance, which means it can break.

Diamonds have four specific cleavage directions, or natural planes of weakness, where their atoms are far apart. These points of weakness are where diamond cutters choose to "cleave" or cut their diamonds.

Besides cleavage direction, the following inclusions could also affect a diamond's durability:

• Feather (generic Gemological Institute of America term for any break in a diamond)
• Cavity (a large or deep opening)
• Chip (a small opening usually on girdle edge)

The inclusions that are unique to a specific diamond are listed on its grading report under "Key to Symbols." Typically, the larger the inclusion, the greater the danger.