The physical properties of gemstone, which are hardness, specific gravity or density and in the way they break or cleave, depends on chemical and atomic bonding within the internal structure of a gemstone. For instance, diamond is the hardest natural mineral and graphite is one of the softest yet, both of them are made from the same element i.e., Carbon. It is due of the carbon atoms bonded together in the diamond with which, it gets such a great hardness.

Hardness: One of the major qualities of a gemstone lies in its hardness, which defines its resistance to scratches. Below is the Mohs’ scale, which was derived by German mineralogist Friedrich Mohs’, for classifying the relative hardness of minerals. Intervals between the number of the scale is not equal. The scale has been given in a descending order in which, Diamond is the hardest & Talc is the softest. The Mohs’ scale:

Specific Gravity: The specific gravity of a gemstone is an indication of its density. It is calculated by comparing the gemstone’s weight with the weight of equal volume of water. The more a gemstone’s specific gravity, the heavier it will be. For instance, a small piece of hematite with an SG of 5.2, will be heavier than a large piece of fluorite with an SG of 3.18.

Cleavage & Fracture: Gemstones can break in two ways. Either they cleave or they fracture. So the way gemstones break, depends on its internal atomic structure. Gems that cleave tend to break along planes of weak atomic bonding. These planes are usually parallel, perpendicular or diagonal to the crystal faces. Gemstones may have one or more directions of cleavage, which can be defined as perfect, distinct or indistinct. When a gemstone breaks but not because of it internal atomic structure, then it is considered as fracture. Fracture surfaces are generally uneven as compared to a cleavage.