One of the most impressive tests for purity of an essential oil can be done by placing a drop on a piece of paper. Pure essential oil will dry without a trace. Only about 700 plants world-wide are considered to be aromatic, and therefore useful for the production of essential oils. While most oils are distilled from the plant material, the types of plant, the unique chemical make-up of its oil, and the location of the oil within the plant determine the extraction process. Expression is the best choice for citrus fruits, because the oil is found in tiny sacs just inside the fruit skin.
Distillation is the most common method.
Most plant oils yield to this process, especially herbaceous plants such as rosemary, and spices such as cinnamon. Most flower essences, such as jasmine and carnation, are too delicate for distillation and a solvent extraction is the only method that allows the recovery of these oils. Solvent extraction is the process of soaking the plant extracts in hexane for a period of time. It is then boiled down to a greasy consistency which is called concrete. This concrete is then dissolved in alcohol and then boiled again to burn off the alcohol to extract the absolute.