If diluting with carrier oil is the crude layer, this is the skilled one.
The blender does not use something odourless. They use a real essential oil, cheaper, with a similar profile, and mix it in at a proportion large enough to lower the cost but not large enough to spoil the smell.
Three classic pairs
Lavender and lavandin. Lavandin is a hybrid, many times more productive and much cheaper. The smell is close but more camphoraceous and less refined. This is the most common blending in the whole trade.
Rose oil and geranium. Geranium contains a lot of citronellol and geraniol, two compounds also found in rose. Blend it in and the proportions still fall within the acceptable range, but the raw materials differ in price by tens of times.
Lime peel and lemongrass. This is where misunderstanding is easy, so it needs care. The two are not rich in the same compound: lime peel oil is mainly limonene, with citral making up only a few per cent; lemongrass is the reverse, with citral making up most of it.
That is exactly why filling lime with lemongrass leaves a very clear trace: the share of citral jumps abnormally compared with the known range for lime peel. It is a blending worth noting in Vietnam because lemongrass is a plentiful and cheap local raw material, not because the two are similar in composition.
Why it is harder to detect
Both ingredients are real essential oils. The paper stain test is useless. The total of characteristic components is still normal.
What changes is the ratio between components, and to notice that you have to know the normal range for the species.
Three signs on an analysis
The ratio between the two main components drifts out of the known range. In real lavender, the ratio of linalyl acetate to linalool usually falls within a fairly stable band. Lavandin has a different ratio and more camphor. A sample labelled lavender with unusually high camphor is one to question.
Marker components of the other species appear. For lavandin, the reliable markers are unusually high camphor and 1,8-cineole — lavandin has plenty of both, real lavender little.
A trap to avoid here: lavandulyl acetate is a marker of the genus Lavandula, not of true lavender alone. Lavandin also belongs to that genus and also contains it, only at a lower level. Using its absence to conclude anything about lavandin is using the wrong marker.
Trace components disappear. Real essential oil has dozens of components below one per cent. They hardly affect the price but they are the fingerprint of the species. Blended goods usually have fewer trace components, because the blender cares only about the big peaks.
The principle
Read an analysis by ratios, not by absolute figures.
A sample with thirty per cent linalool says nothing yet. The question is what that thirty per cent stands beside, and whether the ratios between them fall within the normal range for the species.
This is also why an analysis should be read together with the known range of variation for the species, rather than as a figure standing alone. A supplier who publishes identical figures year after year is something to question, not something to trust.