Numbers may differ slightly among devices used to measure diamonds. This is the nature of instrumentation; none is perfect, only more or less precise and accurate. Fortunately, differences are so slight that they are insignificant in human terms.
Official Numbers
Because of their independence and consistency we consider the diamond’s laboratory report to be the “document of authority.”
Sarin Error
Non-contact scanners like Sarin, Ogi and Helium have a given error. Sarin’s error is ± 0.2 degrees angular and 2.0% linear. When properly calibrated and maintained our onsite reports and laboratory grading reports are consistently within this range, if not closer. If a discrepancy occurs we consider the lab grading report the document of authority (see the below regarding GIA rounding).
GIA Rounding Policy
Our Sarin report may be more technically accurate than GIA reports. This is because the GIA rounds several numbers on their grading reports: Crown angle is rounded to the nearest 0.5 degree. Pavilion angle to the nearest 0.2 degree. Star and lower halves are rounded to the nearest 5%. In deference to the GIA as an institution, we still consider the lab report the document of authority.
Carat Weight
The Federal Trade Commision requires diamonds be weighed to the tenth of a carat but most major labs report to the hundredth of a carat. The AGS actually weighs diamonds to the thousandth of a carat but there is a limit to repeatability and accuracy at three decimal places: Even variations in elevation (which increases the distance from the center of the earth) and in latitude (due to slight error caused by the centrifugal force of the earth spinning) will influence measurements, scale to scale, at this level.
More Information
GIA Research Article on Measurement Tolerances
http://www.diamonds.net/news/NewsItem.aspx?ArticleID=11815
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