Laboratory tier is the trade's informal ranking of gemmological laboratories by the rigour, consistency and market acceptance of their grading. The tier in which a laboratory is placed determines the price premium or discount that its reports carry and the readiness with which the trade will accept its reports as authoritative.

For diamond, the universally accepted top tier is GIA, with a small group of other rigorous laboratories (HRD, GCAL, IGI's higher-tier reports) on a second tier. Below that sit a wide range of laboratories whose grading is observed to be looser by varying margins, from a single grade to two or more on colour and clarity. EGL, GSI, in-house retailer laboratories and various regional labs typically occupy this third tier.

For coloured stones, the top tier is occupied by SSEF, Gubelin and AGL, with GIA Coloured Stone, Lotus Gemology and a small handful of others on a closely adjacent tier. GIT, GRS and several specialist laboratories occupy the next tier, with regional and in-house laboratories below.

Tier placement is not formal but is well understood within the trade and reflected directly in price. The entries on lab-report premium and lab divergence discount discuss the resulting price effects.