Essential oils - essential information

What are Pure Essential Oils ?

I may provide some main criteria for your consideration:

+ Growing and soil quality : 
Materials from pesticide-free soil are the best quality! I know some 
distillers working in these conditions.  Skeptics could add that, this is wishful thinking, pollution is spreading.  I agree, but I believed there are 
still a lot of (relatively) unpolluted places in the world.

+ Harvesting : Performed with hand tools or machinery, but more importance is the exact time of harvesting. The components of essential oils could change depending on the time of harvest, not only 
before or after flowering, but also if harvested in the morning or late in 
the day.

+ Distillation methods : Freshly-harvested materials are used in distillation... 
There are some exceptions where the distillation is easier if the plants are dry (for instance, dried peppermint will be easier to distill, and the distillation time will also be shorter). Ylang ylang needs to be distilled immediately, to avoid fermentation and loss of fragrance. 
The seeds of anise and fennel, stored in good conditions, are easier to keep than the essential oils of them.

Distillation is performed at low temperature and pressure; pure spring water and stainless steel equipment is utilized for optimum cleanliness. 
This will also prevent discoloration of the essential oils: lavender distilled in old copper alembics can turn into a red-colored essential oil.

+  Solvent extraction : The essential oils are 100% pure and not in any way "extended" with cheaper materials 
( relatively small volumes are produced ). Good pure quality essential oils will never be a mass-market line.

How do we know whether we buy good quality essential oils?

+ Certificate of
 Analysis made by an independent organization.

+ Information that should printed on the label :

-       Latin name : The only way to really know which aromatherapy oil is which is, to identify the oils using their botanical (Latin) names. Latin name avoided mislead ( For example, "rose geranium" may sound like a mixture of rose essential oil and geranium essential oil, but in fact rose geranium essential comes from a unique, single species of plant ). Latin name tell you exact properties of the oil.

-       Chemotyping constituents : Some herbs that produce essential oils can produce very different 
essential oils, though they all have the same botanical name, 
for instance basil, thyme or rosemary. 
Thyme can contain a majority of the component linalol, or of thymol, 
or of carvacrol... and thus have different medical actions, so it should be 
specified: Thyme ct linalol).

-       Country of origin :  It sometimes makes a big difference (Lavender - France, Bulgaria, USA...).

-       Extraction methods:  An essential oil obtained with solvents should be described on the bottle as an absolute. An essential oil obtained in synergy with another essential oil (for 
instance Melissa officinalis, Lippia citriodora, Spirea ulmaria, Tilia 
silvestris...) should be described on the bottle as a co-distilled essential oil.

Above information, I had been gathering from a few different sources when studying about essential oils myself. Hope it’s useful.