You cannot tell by looking. The difference between an as-grown laboratory diamond and one treated after growth is recorded on its grading report, and nowhere else you can check without laboratory equipment.
Both are real diamonds. Treatment is not a defect and it is not a substitute stone. But it is a material fact about what you are buying, it affects what the stone is worth, and it is disclosable. This page explains where that disclosure lives and how to read it.
What "as-grown" means
An as-grown diamond leaves the growth chamber with its final color and clarity. Nothing is done to it afterwards beyond cutting and polishing. It applies to both growth methods: a stone can be grown by CVD or by HPHT and be as-grown either way.
What "treated" means
A treated diamond has been through a further process after growth. The common one is high pressure high temperature annealing, which is used to reduce a brown or gray cast that can occur in CVD growth. Irradiation and heating are used to produce fancy colors.
Note the wording carefully, because two things share a name. HPHT is both a growth method and a treatment. A stone can be HPHT-grown, or CVD-grown and then HPHT-treated. Those are different facts and a report distinguishes them.
How common is it
Common enough that it is worth asking about. GIA's own research reports that roughly 80% of the CVD-grown diamonds submitted to GIA have undergone post-growth processing.
Read that number carefully, because its limits matter. It describes stones submitted to one laboratory, not every laboratory-grown diamond in existence, and stones are not submitted at random. It is enough to establish that treatment is ordinary. It is not enough to tell you anything about a particular stone. Only that stone's report does that.
Why you cannot see it
Treatment changes the stone at a level no magnification reaches. A jeweler's loupe will not show it. Nor will a standard thermal diamond tester, which is built to separate diamond from simulants and says nothing about treatment.
Detecting it requires laboratory instruments: photoluminescence spectroscopy, or deep-ultraviolet fluorescence imaging of the kind used in a grading laboratory. This is why the report matters. It is the only access a buyer has to the answer.
Where it is written on the report
On a laboratory-grown diamond report, look at the comments or additional information section, not the four grades. Grading laboratories disclose post-growth treatment there.
The wording is specific and worth learning:
"As grown" or "no indication of post-growth treatment": nothing was done after growth.
Wording noting that the stone may have received post-growth treatment: treatment is disclosed.
The exact phrasing varies by laboratory and by report date.
If a report has no comment about treatment at all, that absence is not the same as a statement that the stone is as-grown. Ask.
What to ask a seller
Three questions, and all three have short factual answers:
Is there a report for this exact stone, and what is its number? Not a report for a stone like it. A report number can be checked against the issuing laboratory's own register.
Does the report state as-grown, or does it disclose post-growth treatment?
Which laboratory issued it? Grading is not uniform between laboratories, and knowing which one graded a stone tells you what the grades are relative to.
A seller who cannot answer these is not necessarily hiding anything. They may not have been told either. That is worth knowing before you buy.
What we do about it
We ask our grower to state, for each stone, whether it is as-grown or has received post-growth treatment, and we print the answer beside the report number on that stone's page.
Where we do not have that answer yet, we say so and show no grade at all. A stone with no published report number on our site is a stone whose report we do not yet hold.
Questions
Is a treated lab grown diamond still a real diamond?
Yes. Treatment changes color, not the material. It is diamond before and after, with the same hardness and the same optical behavior. A simulant such as cubic zirconia or moissanite is a different material entirely, and is a separate question from treatment.
Does treatment make a diamond worse?
It makes it different, and generally less valuable than an equivalent as-grown stone, which is why disclosure matters. It does not make the stone fragile or unreal. Whether the price reflects the difference is the thing to check.
Can a jeweler tell me if my diamond was treated?
Not by examining it, unless they have laboratory instruments. They can read your grading report, which is where the answer is recorded.
Is HPHT a growth method or a treatment?
Both, and this is the most common confusion in the category. A diamond can be grown by HPHT, or grown by CVD and afterwards treated by HPHT. The report distinguishes them.
What if my report does not mention treatment?
Silence is not a disclosure. Ask the seller which laboratory issued the report and whether the stone is stated as-grown.