Essential oils are highly volatile substances, which readily evaporate. About 20% of all medicinal herbs give their healing properties to essential oils. There are approximately 300 essential oils available commercially however, only 50 to 100 have properties suitable for use by the aromatherapist. Although technically classified as oils, they do not leave a permanent mark on paper and have a consistency closer to water and are not greasy. Essential oils are all inflammable. They do not dissolve in water and are slightly soluble in vinegar. They dissolve fairly well in alcohol and mix well with vegetable oils, fats and waxes. The oils are found in tiny oil glands or sacs, which are concentrated in different parts of the plant. They can be found in flowers e.g. rose, neroli, ylang ylang, chamomile, jasmine; in leaves e.g. eucalyptus, petitgrain, basil, rosemary, clary sage; in roots e.g. vetiver, ginger; in grasses e.g. lemongrass, palmarosa; in wood e.g. sandalwood, cedarwood, rosewood; in fruit e.g.lemon, tangerine, bergamot, grapefuit, orange, in seeds e.g. caraway, celery, coriander; in nuts & berries e.g. juniper, cypress, nutmeg, in bark e.g. cinnamon and in gums e.g. frankincense. Sometimes the oil is yielded from more than one part of the plant as is the case with lavender, where the essential oil is derived from both the flowers and the leaves. The orange tree yields three different essential oils, neroli from the orange blossoms, petitgrain from the leaves and orange from the fruit. Essential oils vary considerably with regard to the amount of essential oil they yield and it is understandable that this factor will have an affect on the market price of the different essential oils. Not all plants produce essential oils but in the plants that do they play an important role in the propogation of the plant, attracting bees etc and in defense.
Obviously the more oil glands present in the plant the greater the yield of oil and the lower the price. For example, it will take approximately 50 lbs of eucalyptus to produce one lb of essential oil, about 150 lbs of lavender to produce one lb of essential oil, about 500 lbs of rosemary to produce one lb of essential oil and anywhere between two to three thousand pounds of rose petals to produce one pound of essential rose.