When grading diamonds, proper tools such as proportion analysers to determine cut and master stones or a colorimeter to determine colour should be used.
However, if you’re these guys, then such tools aren’t necessary. In fact, you can grade diamonds when the diamond is set in a ring. Amazing!
To be fair, it does say “Graded As Mounting Permits” at the bottom, but one has to wonder how they arrive at the cut grade. Maybe they hold it up to the ceiling and use an Ideal-Scope?
AUD 
August 14th, 2007 at 5:49 am
According to the GIA and CIBJO Blue Book you should not grade diamonds without a master diamond set. Color meters are not accurate and are affected by florescence and secondary colors. According to Sarin the DC3000 Colorimeter grades loose and mounted diamonds equally well. GIA and AGS do NOT use colors meters. And to my knowledge it is a retail tool only. If you are buying a stone graded on a color meter it could be two colors lower.
August 14th, 2007 at 6:09 am
Model DC3000
Diamond
Colorimeter
User Guide
This is from my sarin user guide
Fluorescence is far more complex than most people
realize. When a diamond is fluorescent it typically
appears whiter when exposed to sunlight than under
incandescent light. This is because UV light (less than
380 nm in wavelength) contained in natural sunlight
stimulates a fluorescent diamond to typically emit blue
light thus making the diamond appear whiter.
Most fluorescent lamps generate a small amount of UV
in their output which can also make a diamond viewed
under such a lamp appear whiter.
The fluorescent phenomena can be observed by holding
something fluorescent (most white paper for example)
in the Colorimeter UV viewing chamber after pressing
the UV key. The UV chamber injects long wave UV
(365 nm) into the area above the black glass window.
Because most visual diamond grading is done using a
fluorescent lamp the grading results for fluorescent
diamonds can change depending on the characteristics
of the lamp as well as how close the diamond is held to
the lamp.
Both visual grading and Colorimeter grading of
strongly fluorescent stones can give misleading results
and some caution is advised whenever you attempt to
grade a strongly fluorescent stone.
• Fluorescent stones can give misleading results.
Always use the UV viewing chamber to observe
if a diamond is fluorescent. Remember, visual
grading of fluorescent stones is very dependent
on the light source being used.
Do NOT rely on the results!!!!!!! and careful of labs that do.