The main factors to look for to ensure you get a good quality essential oil are:
That at the plant has been grown specifically for aromatherapy, it has been grown in the right environment, it has been nurtured without the use of pesticides or soil enhancers, the plant has been harvested at the right time to get the best quality essential oil and not necessarily the most, that the distillation process is completed making sure that all of the relevant chemical components are given the amount of time necessary for them to be extracted from the plant material, that no adulteration has taken place – nothing added or taken away.