As complete as this list may appear, this is only a sampling of the oils. There are many good books on the subject of Aromatherapy. I encourages you to educate yourself as much as possible. This is the volatile substance extracted or distilled from Botanicals that contains concentrated and pure plant properties. Although some plants tend to be higher in natural oils than others, essentials are rarely oily in their consistency.
Most essential oils are rather thin and evaporate easily.
Resin essentials like Frankincense or myrrh may be thick and have a tendency to harden. The essential oil carries the fragrance of the plant. Some plants, like the orange tree have different essentials oils produced from different parts of the plant. For example, we get orange oil from the rind of the fruit. The fragrant essence of neroli comes from the flower and Petitgrain is extracted from the leaves.