Specific gravity : This compares the weight of a certain volume of oil to the weight of the same volume of water. Specific gravity is normally expressed as a decimal ( the specific gravity of water being 1.0). Most essential oils,except those from woody plants, have a specific gravity less than 1.
e.g. lavandula angustifolia 0.878 - 0.892 ; citrus reticulate: 0.854 - 0.859
Specific gravity is most accurately measured with an electronic meter, although it can also be measured mechanically with a hydrometer.

Optical rotation : Depending on the arrangement of the atoms in their molecules, certain substances have the ability to rotate plane polarised light. The total optical rotation for an essential oil is the summation of the rotation of its chemical constituents. Optical rotation is measured by a polarimeter.
e.g. rosmarinus officinalis -5 - +24 ; citrus bergamia +12 - +24

Refractive index : When light rays pass from one substance into another e.g. from air into a liquid such as an essential oil, the rays are bent or refracted. The amount they are bent depends on the density of the liquid and is given by the refractive index of the liquid. Refractive index is measured by a refractometer.
syzgium aromaticum 1.528 - 1.537 ; citrus reticulate 1.475 - 1.478 ...................click here or on the arrow below for more