The nature of an essential oil varies from plant to plant. Depending on the species of plant, essential oils are extracted from flowers, leaves, roots, wood, bark, resin or fruit, using a variety of methods to capture the scented particles. The most common way of obtaining the oil is by steam distillation. This involves putting the plant material in a large, closed container, and forcing steam through it. The heat and the pressure release the tiny droplets of essential oil from the plant. Then a cooling process turns the steam into liquid.
Naturally, some plants yield lots of oil easily, and others have more labor-intensive processes, yielding lower amounts of essential oil. Damascene rose is the most extreme example: It takes approximately 4000kg of rose flowers to produce 1kg of essential oil. You now understand better its high price tag (5000Eur/kg)....
The composition of an essential oil can vary depending on the time of day, season, geographic location, method and duration of distillation, year grown, and the weather, making every step of the production process a critical determinant of the overall quality of the product.