Diamond Shade and BGM: What to Check Before Buying
Learn what brown, green and milky mean, why a report may not settle the question, and how to check the diamond you are buying.
Reviewed by Adnan Ali, Graduate, GIA Diamond Grading · Updated 2026-10-18
BGM is shorthand used by some sellers for brown, green and milky. Brown and green refer to a colour cast; milky refers to a hazy appearance. For you as a buyer, the question is simple: does the actual diamond look clean, crisp and the colour you expected?
What you might notice
| Appearance | What it looks like | What to ask |
|---|---|---|
| Brownish shade | A brown or warm cast in the stone. | Does the cast remain in neutral light, away from coloured surroundings? |
| Greenish shade | A greenish cast in the stone. | Is it in the diamond, or a reflection from the lighting, clothing or background? |
| Milky / hazy | A fog-like look with less crispness through the stone. | Has it been cleaned, and does the haze remain in ordinary lighting? |
The brown photo is a strong colour example, not a matched comparison of subtle shades. Deliberately coloured fancy diamonds are a different buying choice; a visible colour is not automatically a fault.
Why the certificate does not settle everything
Most standard reports do not have a BGM pass/fail box. Colour grades, clarity features and report comments may still give useful information. A high clarity grade, no comment or a seller’s “no BGM” label does not replace a view of the actual diamond.
How to check the actual stone
- Ask for a video of the clean diamond in neutral light, with a plain background.
- Compare stones in the same lighting and at the same scale.
- Look for a persistent colour cast or a foggy appearance as the stone moves.
- Ask the seller to confirm in plain words whether there is any noticeable brown/green cast or milkiness, and explain the viewing conditions.
- Check purchase terms before paying if you have not seen the stone in person.
Fluorescence and price
Strong fluorescence does not automatically mean milkiness, and a diamond with little fluorescence can still look hazy. Judge the actual appearance. There is no universal BGM discount: a low price is a reason to compare carefully, rather than proof that something is wrong. If a shade or haze bothers you, a discount may not make it the right diamond for you.
Sources: GIA: colour and clarity quality factors · GIA: fluorescence, contrast and transparency research · GIA: cloud inclusions and appearance · GIA: fancy-colour diamond reports