Essential oils are not natural. They are refined products extracted from plants, and are highly concentrated. They contain many of the volatile constituents of the original plant material, and some additional components which are either created by the process used to extract them, or are added deliberately as solvents.
True essential oils are unadulterated, and so are produced either by distillation or expression (citrus oils are pressed from the peel), although the term is often inaccurately applied to all aromatic extracts including concretes, resinoids and absolutes which are only partly composed of essential oils.
The majority of essential oils have traditionally been produced by steam distillation. Steam passes around the plant parts drawing off the essential oils. The steam is collected and condensed, giving water in which the essential oil separates at the top or bottom. The water is drawn off and can be used again, producing second, third etc distillations. Oil from the first distillation gives the best quality. The resulting oil can be separated into different fractions (e.g. yellow, white and brown camphor). Improved distillation techniques include turbo distillation, hydrodiffusion, vacuum or dry distillation, continuous distillation and molecular distillation. Essential oils are usually liquid, and dissolve in pure alcohol, fats and oils but not water. They evaporate on exposure to air without a residue.
Concretes and resinoids are prepared by using hydrocarbon solvents. Most concretes are about 50 percent wax, and solid with a waxy consistency. They are more stable and concentrated than essential oils. Absolutes are made from concretes by extraction using ethanol as a solvent, in which the wax is only slightly soluble. Small amounts of alcohol remain, making the absolute unsuitable for therapeutic use. Recently, techniques using liquid carbon dioxide as a solvent have been successfully developed, producing oils with no trace of solvent or involatile material.
The chemical constituents of an oil, and thus its properties and usefulness are heavily influenced by the exact species of plant, its growing conditions and even the time of day that the crop is gathered, not to mention the extraction process. This means that you have to be careful in choosing oils - first of all to make sure that you know the exact plant (using the Latin name which classifies it botanically). Even then, the same oil from different suppliers will smell subtly different and is likely to vary over time.
Since essential oils are volatile, they must be kept away from sunlight and high temperatures. Vibration should be avoided too (some people are even concerned about loud music).