Essential oils come from all over the world.  China, Europe, South America, Africa, Middle East and other countries.  With such a variety of origins, knowledge about how to judge the quality of essential oils is extremely important. 

100% therapeutic essential oils is often the most misunderstood phrase in the world of Aromatherapy.  Many people mistake Purity for Quality.  Plants can be distilled over and over, some times 9 or 10 times.  Each time it is considered "100% Therapeutic Oil", which is technically true.  However each time the plant is distilled it loses it "Quality" and becomes less and less effective.  Take Peppermint for example.  When people think of peppermint they think of the cool, fresh, minty smell-which is actually the 2nd distillation and the most popular.  The first distillation of peppermint actually has an herbal smell to it.  For the most effective and best essential oils you need to remember to look for Quality as well as Purity.

The time and temperature during distilling also plays a huge roll in quality.  When Lavender reaches 280 degrees it will fracture the oil.  The fragrance will still be there but the oil is ineffective or even toxic. 

It is considered to be adulterated, when essential oil is diluted in carrier oil or with synthetic oil and not indicated on the label, Also blending one batch of essential oil, such as Lemon from the USA with another batch of Lemon from a different crop is considered adulteration of the oils.

Adulteration of the oils, purity and quality cannot be detected by sight, smell, or taste.  The Gas Chromatography Test (GC) and the Mass Spectrometry (MS) are some of the  ways to judge quality and purity of the essential oils.  They are other tests needed also, phlates tests, parts per trillion test. ect.

A pure essential oil will last centuries if properly care for.  Essential oils need to stay out of direct sunlight and stored in a dark bottles.  It is best to keep them in a cool place.  A few years ago scientists excavated a tomb and found large vats of essential oils and learned after testing them, that they were still top quality.  Like wine, some essential oils actually get better with time.  If you want a great Sandalwood, Patchouli, Birch or Vetiver (among others), let it age for a year or two before use.  However, cold pressed oil such as the citrus oils will only last about 3 years before they will go rancid.  For long term storage be sure your citrus oils are distilled. Not cold pressed.

Keep your essential oils away from computers and other large appliances.  These appliances can change the electrical frequency of the oils.

It is best not to use a metal funnel when bottling your oils, a metal diffuser or store in a metal container.

Here are some things to consider if you are concerned about getting the very best oils.  None of the following tests are for sure, and there are lots of ways to adulterate essential oils that can't be detected at home, still, here are some things you can try.

Blotting paper test. Put a drop onto blotting paper or a coffee filter and leave it overnight.  If a visible oil stain appears the next day, the oil may possibly be extended with carrier oil.  Dense, viscous oils like patchouli, vetiver will leave stains, as well as some cold pressed (citrus) oils, otherwise this is a good indicator.

Trial by Water.  Put a few drops in a clear glass vial, add a little water and shake.  Wait a few minutes.  If the mixture is cloudy after it's had a chance to settle, there may be alcohol or some other adulterant in the oil.   This isn't good.  Cloudy doesn't mean opaque or translucent, every oil is not transparent by nature. 

Nose test.  Smell it.  The more experience you have with more oils, the better you'll be able to smell good oils from bad oils. Be assured though the nose test is never the sure fire test.  Genuine essential oils are not "fragrance oils" and lots of them smell different from what we might initially expect.  Some smell downright icky, that's their nature.  At the same time some adulterants smell lovely.  Ask yourself these questions, "Does it somehow smell fake"?  Is it flabby or weak"?  "Is it a full smell or is something missing"?

Miscellaneous Tests.  There are also some non-sure fire tests for specific oils:  apparently the very best quality vetiver sinks in water.  Genuine Rose Otto turns solid in the refrigerator, and the best Rose Otto should be partially solid even around room temperature.

Whatever company you choose, make sure they are having their oils tested, every batch that they sell.

Testing insures quality!