The Mohs scale does not increase at designated equal stages, for example Aquamarine ranks eight, Ruby nine, and Diamond ten yet Ruby is twice as hard (approx) as Aquamarine and Diamond is four times (approx) harder than Ruby as shown in the graph showing the absolute harness of stones .It is worth remembering that although a substance may be hard, it may not be strong, for example, platinum rates four on the Mohs scale yet glass rates six. Glass is the harder substance, however it is also extremely brittle. This also can be true of many stones.

Cleavage plains

The way a stone is formed and the type of crystal system they are made up of, dictate its strength. For example Tourmaline has a Trigonal crystal system meaning it has basal cleavage. Basal cleavage means the stone can crack parallel to the base of the stone if it is banged.
All stones are made up of crystal systems giving each stone a distinct set of properties. All stones can be broken if hit in the right place, even diamond, when hit in the right area.

For more information on specific stones click the links below,

Garnet Amethyst Aquamarine Diamond Emerald Pearl & Moonstone Ruby Peridot Sapphire Opal & Tourmaline Topaz Lapis Lazuli & Turquoise Tanzanite