Essential oils are concentrated plant essences. Although they are called "oil," this is something of a misnomer because most essential oils are not in fact oily, unlike vegetable oils which have been expressed from seeds or plant nuts (such as sesame, sunflower, peach kernel, and sweet almond). A few essential oils are rather viscous and others are fairly solid. Most, however, are watery, with lavender, lemon, and eucalyptus being classic examples. The color of essential oils varies, tending to be clear or yellow, although carrot is orange, spikenard is often green, and chamomile german a beautiful deep blue. Of course, essential oils are characterized most of all by their individual aroma - hence the term "aromatherapy."

Essential oils are derived from relatively few plants and, depending on the variety, only from particular parts of that plant. These include the leaves, roots, buds, twigs, rhizomes, heartwood, bark, resin, flower petals, seeds, or fruit. In some cases the whole of the plant that grows above the ground is used, as in peppermint, for example. There is a huge variation in the price of different essential oils reflecting, above all, the volume of material available, but also transportation costs and growing conditions in any given year. It takes vast quantities of hand-picked, tine jasmine petals to produce a few drops of jasmine oil while tea tree oil can be produced much more cheaply using mechanical methods to harvest the leaves of that plant. All essential oil-producing plants are different in their yield, as well. Clary sage, for example, yields 0.3-1% essential oil, while clove yields 10-15%. From the same volume of material, then, clove produces up to forty-five times more essential oil than clary sage.

Throughout the world, there are approximately 3,000 essential oils, most of which are used only in the local regions in which they are found. About 300 are in more general use, and are traded worldwide. It is helpful to know where a particular oil comes from because, for example, geranium oils grown in Egypt, Chine, or Madagascar each have their unique characteristics. Also, the same species of plant will produce an oil with different properties depending on whether it was grown in dry or damp earth, for example, or at high or low altitude, or in a hot or cold climate.

Some essential oils contain hundreds of biochemical components, while others have only a few. And some components are present in quite large proportions while many others are present only in traces. The analysis of essential oils is still at the stage where we cannot yet say exactly how many components are in any particular essential oil, because all small traces have not been identified. Indeed, they cannot be registered because the means of analysis are not yet available. Each essential oil has a unique fingerprint, and we cannot fully identify that fingerprint because parts of it are not yet "on file" - the components are simply unknown. As methods of analysis become ever more sophisticated, and as the scientific world's catalog of components enlarges, more and more ingredients will become identifiable.

The therapeutic action of essential oils is usually attributed to the naturally occurring chemicals found within them. Within the essential oil of yuzu (Citrus junos), for example, there are known to be 124 compounds, composed of 44 alcohols, 26 hydrocarbons, 12 esters, 9 ketones, 14 aldehydes, 3 phenols, and 16 miscellaneous others. Other terms in the vocabulary of essential oil analysis include terpenes, coumarines, acids, esters, and sesquiterpenes. The way these various natural chemicals interact with each other gives an essential oil its unique qualities and, some would say, explains its therapeutic properties. Alcohols can be shown to be antibacterial, for example, and coumarines sedative.

Various methods are used to establish the compounds in essential oils. The best form of analysis involves using several procedures in conjunction to establish the chemical components and their purity. Capillary gas liquid chromatography establishes the chemical components, while optical rotation establishes whether the essential oil has the correct optical activity - measured in terms of percentages of blend to right or left, or "dextrorotatory" and "laevorotatory" respectively. Also, the specific gravity of the material is measured to establish that it has the correct weight, and the refractive index establishes whether the material reflects the correct angle of sodium light. Together, these tests can establish the purity of the essential oil.