Diamonds are grading according to the world standard scales of the 4 C's as well as additional characteristics on the fluorescence, polish, symmetry and facet angles and ratios. Some of these measurements are completely objective, for example carat weight and dimensions, some are measured and graded by the lab's tolerance or research and some are more subjective to the specific grader.
A gemological lab is a professional, controlled authority that uses tools to test the authenticity and quality using a set of tools to independently measure and grade the stone at hand.
A gemological lab is independent from the owner, buyer and seller of the diamonds, gemstones and jewelry being sent for inspection and so can offer impartial grading which is especially important when considering the more subjective specifications. Independent grading is especially important when comparing to in-house appraisals or gemological reports issued by the jeweler or diamond trader as specifications may be over-stated and thus diamonds and jewelry over-valued.
At TIG we go a step further trying to eliminate every possible grader-specific potential bias or opinion by championing the most advanced instruments to accurately and consistently grade. However, some elements of grading are subjective to the grader and so may be upgraded or downgraded if a borderline grading is encountered. In such cases, a second and third opinion will be sought within the lab.
Different labs use different tools, have slight discrepancies with grading tolerance and still have elements that are subjective to the grader. As research and technology advance, these differences will be reduced and this is something at TIG we strive toward.
The main difference between a diamond certificate and a jewelry certificate is whether the report focuses on the granular details of the diamond or gives a higher-view report on the end product. When a diamond is loose, it can be weighed and every angle of the stone can be measured. Once in a setting, the main diamond weight is calculated by formula and the cut grade is based on the face-up view (the most important view) only. Similarly, the diamond color and clarity are graded based on the available view once the diamond is set and so some limitations will apply. Jewelry certificates, however, will also include details of the total carat weight, including side stones, the metal type, purity, stamps and weight - information not provided on a diamond report.