A gemstone certificate from a recognized laboratory is the closest thing to a passport for a stone. Buyers frequently request certificates, but fewer actually understand what the individual fields mean and how to read them. A certificate that goes unread offers no protection.
Report Number
Every certificate carries a unique identification number. This number ties the physical stone to its laboratory record and allows anyone to verify the certificate's authenticity through the lab's website or QR code. At GTCL, our report numbers follow the format GTCL-YYMMXXXX, encoding the year and month of testing.
Stone Identification
The certificate names the stone species (the mineral family, such as corundum) and the variety (the gem name within that family, such as ruby or sapphire). This distinction matters: a "blue stone" on a sales receipt could be anything from sapphire to blue glass. The species confirmation is based on optical and physical testing, not visual appearance.
Weight in Carats
Weight is recorded in carats (1 carat = 0.2 grams), measured to two decimal places. This is the primary unit for pricing and comparison. A stone described verbally as "about two carats" might actually weigh 1.85 ct or 2.15 ct. The certificate removes guesswork.
Colour, Transparency, and Cut
Colour is described using standard terminology: hue (the basic colour), tone (light to dark), and saturation (intensity). Transparency ranges from "transparent" through "translucent" to "opaque." Shape and cut describe the stone's outline and faceting style. Together, these three fields document the stone's visual character objectively.
Refractive Index and Specific Gravity
These are the diagnostic measurements that confirm species identification. Each gem mineral has a documented RI and SG range. If a stone presented as sapphire shows an RI of 1.54 (instead of 1.762–1.770), it is not sapphire. These numbers are the scientific backbone of the certificate.
Treatment Disclosure
This field records any treatment detected: heating, oiling, filling, diffusion, irradiation, or coating. Treated stones are not necessarily less desirable, but an undisclosed treatment dramatically affects value. A heated sapphire might sell for 30 to 50 percent less than an equivalent unheated stone. The certificate ensures you know what you are paying for.
Origin (Where Stated)
Geographic origin affects pricing significantly. A Burmese ruby commands a premium over a Thai ruby of similar quality because of market perception and historical reputation. Origin determination involves trace element analysis and inclusion study, and labs will only state an origin when the evidence is conclusive.
Reading a gemstone certificate carefully before making a purchase is the single most effective way to protect yourself in the gem market.
Need a gemstone certified?
Contact GTCL
