Since essential oils aren't just laying there in the forest in puddles ready for us to bottle them, essential oils have to be created from various plant parts like flower petals, needles and bark from trees, resin, roots, seeds, and more. There are several steps in this process, all of which affect the quality of the end result: your essential oils.

Growing conditions can affect the quality of the plant, and therefore the therapeutic value of the essential oil. Climate, temperature, quality of soil, amount of rainfall, altitude - all these factors come into play.

Once the proper growing length has been determined, the plants are harvested (or in the case of resins like Frankincense, they are gathered). How it was harvested (or gathered), how it was stored (cool temperatures? blazing hot sun?), and how long it sits around (two hours? two years?) all affect the final product.

Next, we have extraction, which is done in a number of ways:

• Steam distillation is the most common method used to process essential oils. This involves steaming the plant matter of choice (flowers, roots, leaves, etc.) for a determined amount of time, under specific pressure, and at a certain temperature to release the oils from the plant. The collected oil is officially an "essential oil.”
• CO2 extraction is similar to steam distillation, only liquid CO2 is used instead of water. After the liquid CO2 and the plant matter combine, the CO2 is turned back into a gas, leaving the plant matter and essential oil behind. This provides a different, arguably more pleasant aroma than you typically get from steam distillation, although shelf life may be shortened.
• Cold pressing involves mashing citrus rinds (bergamot, lemon, etc.) in water. The oil released is separated from the pulpy water and collected as an essential oil. Because pesticides are sprayed right onto the rinds, it's important to seek organic options for cold pressed essential oils.
• Solvent extraction is where plant material, usually flowers, is mixed with a solvent. Traditionally, aromatherapists have generally avoided using absolutes (not true essential oils) due to the amount of solvent left in the final product, but modern ways of making absolutes have allowed for only trace amounts to remain. Aroma is typically less pleasant, and the quality lesser, compared to other extraction methods. These are often labeled as an "absolute."

Plant parts used during the extraction process, the type of distillation equipment used, temperature selected for distillation, the length of time plants are being extracted, and the exact pressure used during extraction are all reflected in the essential oil.

After the essential oils are processed, they are stored until ready to be sold in bulk to hundreds of companies all over the world. The length of time held, the temperature in the room (or is it outside?), the container used to store the essential oil (is it even covered?) all affect the essential oils bought by companies. What happens to them next is up to the company.

Once you’ve made your purchase from the company, how you store them can determine how much longer they last before they expire.