Pedanius Dioscordes was born in Anazarbus, a small city northeast of Tarsus in the Roman province of Cilicia (now we call it Turkey) around the times of Nero and Vespasin. He was a surgeon in the Roman Army and traveled with them to Italy, Greece, Northern Africa, Gaul, Persia, Egypt and Armenia. His travels gave him the oportunity to study a great many medicinal plants, animals and minerals. Dioscordes started "De Materia Medica" which was a list of known medicinal plants of the Roman Empire. It was eventually finished roughly 20 years later, around 50 AD.
Essential oils are volatile liquids which are distilled or pressed from plants and plant matter (even parts of plants such as seeds). One of the factors which determine the purity of an essential oil is it's chemical constituents. These constituents are affected by many different things including but not limited to: the plant parts used, soil conditions, fertilizer used, geographic region, climate, altitude, season of harvest and distillation process.