Almost everything written about choosing essential oils is written as though the author owns a still. Almost nobody selling to you does. What follows is the honest version from a step back down the chain: what I actually ask, what an answer is worth, and where the limits of my knowledge sit.
Where I sit in the chain
I buy through established wholesale suppliers here in Australia. I do not import directly and I do not distil anything myself. For a business this size that is a deliberate choice: the chain stays short, I am dealing with people I can pick up the phone to, and the material has already passed through someone who does this full time.
The trade-off is that I am relying on my supplier's word for things I cannot personally witness. So the whole job becomes working out whose word is worth relying on.
The three questions I will not buy without
Which species, in full. A common name is frequently useless. "Frankincense" covers about twenty different trees with genuinely different chemistry, and serrata is not sacra. "Eucalyptus" covers hundreds. "Cedarwood" spans several unrelated genera on three continents. If a supplier cannot give me a binomial, the conversation is already over.
Which country it was grown in. Note the wording. Not where the plant is native, not where the company is registered, not where it was bottled. Where the crop grew.
How it was extracted. Steam distilled, cold pressed, solvent extracted, CO2. These are not interchangeable and they produce chemically different products from the same plant. It is also where a lot of quiet nonsense lives, because a distilled oil and a solvent extract of the same resin can be sold under one name while behaving completely differently.
Native range is not supplier origin
This is the mistake I most want to avoid making in my own writing, and it is the one I have had to correct.
A plant being native to somewhere tells you nothing about where a given drum came from. Boronia is native to Western Australia and much of the world's supply is Tasmanian. Jasmine is grown in India and Egypt and the two smell measurably different. Blue gum is a Tasmanian tree and most blue gum oil is distilled a very long way from Tasmania. Saying "native to X" when you mean "sourced from X" is not a small slip, it is a different claim entirely, and once it is on a page it tends to stay there.
What a GC report can and cannot tell you
Gas chromatography separates an oil into its constituents and tells you what proportions they sit in. It is the single most useful document in this trade and it is not magic.
It will readily show you a profile that does not match the species claimed on the label, and it will usually catch crude bulking with a cheap carrier or a different oil. What it does not do on its own is distinguish a naturally occurring compound from a synthetic copy of the same compound, because chemically they are the same molecule. Detecting that takes chiral analysis or isotope ratio work, which is specialist testing that sits well beyond the scale most small sellers operate at.
So a GC report is strong evidence and not a guarantee. Anyone presenting one as proof of purity is overselling what the document does.
Documentation, honestly
Here is where I would rather be plain than impressive. I do not hold a certificate of analysis or a batch report for every oil on the shelf. Most small Australian retailers do not, and any that claim otherwise are worth a second look.
What I can do is ask. If you want documentation on a specific oil, a certificate of analysis or a compositional report for a particular batch, tell me which one and I will chase it up with the supplier. That offer stands for anything in the range, and I would rather do the work than have you take my word for it.
Batch variation is not a fault
Worth knowing before you read any report. Natural materials move, sometimes a lot. Published lot analyses of frankincense serrata have shown its main constituent ranging from around forty per cent to over seventy in material from a single supplier. That is a plant responding to a season, not a supplier cutting corners.
It does mean a batch number is worth more than a brand name, and that two honest bottles of the same oil can smell noticeably unlike each other. A supplier who acknowledges that is more credible than one whose product is identical forever.
The tells that cost nothing to check
Price is the most reliable single signal there is. Some materials cannot be cheap. If boronia, rose otto, jasmine or sandalwood is affordable, the question is not whether something is wrong but what.
After that: does the label carry a binomial, a country and a batch. Does the range include every expensive material at a uniform price, which is a strong sign that nothing on the shelf is what it says. And does a listing lean on research that belongs to a different product, the way frankincense oil is routinely sold on studies done on frankincense resin.
The most useful question is not "is this pure". It is "what exactly is this, and how do you know". A supplier who enjoys that question is the one to buy from.
What I actually look for
Consistency between batches. Suppliers who can tell me where a material was grown without having to go and invent it. And a willingness to answer questions rather than deflect them, because the deflection is the answer.
If I cannot get a clear response about what something is or where it came from, it does not go on the shelf. That is the whole policy, and it is why some things you might expect to see here are not here.