100% pure essential oils are usually steam-distilled plant essences, that is, the extracted and concentrated juices from flower petals, seeds, fruits, fruit skins, stems, leaves and/or roots.

I say 'usually' because there is the possibility of lax methodology or outright chicanery. What are sold as essential oils can be lab-made blends of scents in carrier oil. These will be cheap, and will not retain an intact scent after they are opened. Real, pure essential oils, if kept stoppered in a cool, dark place, will retain their scent for years with very little degradation.

Sometimes essential oils are blended with a carrier oil, like almond oil, and the essential oil, such as rose or oregano. This will happen because the 100% pure oil is either considered too strong (oregano) for the general public, or it is too expensive to produce (rose absolut). They are often labeled '100% pure' but you have to look closely to see the caveat '20% volume in a carrier oil', or similar. I consider this to be a questionable practice. Of course, blending strong pure essential oils with a carrier oil is a good idea, especially for use on the young, the elderly, or the infirm; this can be done by the informed user. Pre-diluting the oil prior to sale, while still exhibiting the large-print '100% pure' label, is misleading.

Essential oils do have actions upon the body, mind, emotions, and spirit. Tea tree oil is an excellent anti-bacterial and anti-fungal. Basil and Orange are very uplifting, and is used as an anti-depressant. Lemon is astringent and uplifting. Lavender helps heal skin, is calming, and will promote sleep.

You can perform an experiment with any one of these oils to satisfy yourself that they work. Sprinkle a few drops of orange essential oil in a place used by someone who is often down or depressed. Notice if anything changes. Try a drop of lavender on the top of the head of an excited pet or a cranky baby. See if they settle. Use a drop of basil oil on your forehead before a test or exam. Record your experience.

I have an extensive collection of essential oils with which I have doctored myself and my family. My current favourite is 'Medieval Thieves' blend of cloves, lemon, cinnamon, eucalyptus and rosemary. A drop, rubbed between the hands, appears to provide anti-bacterial and anti-viral protection when handling money or shaking hands. Pure essential oils are an excellent adjunct to conventional medicine, and they are valuable on their own as a first line of defense.

Hope this provides a starting point for your research!