Hardness, specific gravity and the way they break (Cleave) are the physical properties and these are depend on chemical bonding and the atomic structure within the gemstone.

The softest natural material is graphite and the hardest is Diamond but both are made out of carbon. Only the difference is bonding way and it gives a particular hardness to the gemstone.
Hardness may be measured by how well a gemstone resists scratching. Every stone can be classified using Mohs’ Scale of Hardness. It gives every mineral a value from 1 to 10. Intervals between numbers on the scale are not equal.

Specific Gravity is an indication of its density. By comparing the stone’s weight with the weight of an equal volume of water Specific Gravity can be calculated. The greater a stone’s Specific Gravity the heavier it will feel.

Substance (weight) in air 5.2 g
Substance (weight) in water 3.3 g
Difference = Volume 1.9 g
Density = weight on air/Volume = 5.2/1.9 = 2.7 g/cm3

Gemstones may break either cleave or fracture. The way they break depends on their internal atomic structure of the gemstone. Gemstones that cleave tend to break along cleavage panes (panes of weak atomic bonding). These panes are generally parallel, perpendicular, or diagonal to the crystal faces of the gemstone and may have one or more directions of cleavage. Cleavage is used to devide large crystals or remove faulty pieces.The largest diamond of gem quality ever found, the Cullinan of 3106 carats, was cleaved in 1908 into three large pieces which were then cleaved into numerous smaller pieces. Today, small pieces are usually sawn in order to avoid unwanted cleavages.
The cleavage can be defined as Perfect, Distinct and indistinct.

When a gemstone breaks along a surface that is not related to its internal atomic structure, is called fracture. These fracture surfaces are usually uneven. It can be conchoidal shell-like), uneven, smooth, fibrous, splintery, or grainy. Sometimes the type of fracture helps to identify a mineral. Conchoidal fracture is, for instance, characteristic for all quartz and glass-like minerals.

The Carat, Grain, Momme and gram are used as units of weight.

The name derived from the seed Kuara of the African Coraltree or from the kernel (Greek keratition) of the Carob bean. The metric carat (mct) 200 mg or 0.2 g. Carat is sub divided into fractions (e.g. 1/10 ct) or decimals (e.g. 1.25 ct) up to two decimal places. Small diamonds are weighed in points. [e.g. 1/100 carat (0.01 ct) ]. Following table illustrates diameter and carat weight for diamonds, cut in brilliant cut. Gemstones with different density and different cut have different diameters.

The price of a gemstone is indicated as “per carat”. By calculating the real weight, receives the price per piece of the gemstones. The carat price increases with the size of the gemstone in small increments. When a one-carat piece costs $1000, then two-carat piece is not worth of $2000, but it may be $ 4000 or more.

Gold smiths are used another unit called Karat, and it is not this weighing carat unit. The higher the Karatage, the higher the content of gold in the piece of jewelry and the weight can be variable.

This unit is used for measure less precious gemstones and especially for rough stones.

This unit formerly, used to measure weight of pearls. It is 0.05 g or ¼ ct. Today the carat is the unit that measure weight of a pearl.

The old Japanese measure of momme (=3.75 g =18.75 ct) for cultured pearls.