Three views of a 391.20grain naturally colored, natural blister pearl.Front (1), back (2) and side-on (3).
— Sturman, N. A Large Naturally Colored Natural Blister Pearl
My recent jewelry interest has me thinking about pearls in-depth, specifically natural (non-curated) pearls. I didn’t expect that well-formed natural pearls were quite as rare and expensive as they’ve turned out to be, and in conversation was asked whether natural and cultured pearls can be differentiated by physical analysis. Turns out they can be told apart relatively easily. Wikipedia’s thorough article on Pearls explains the examination process:
When a cultured pearl with bead is X-rayed, it reveals a different structure to that of a natural pearl. A beaded cultured pearl shows a solid center with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings. A beadless cultured pearl (whether of freshwater or saltwater origin) may show growth rings, but also a complex central cavity, witness of the first precipitation of the young pearl sac.
‘Classic’ natural pearl structure. A dark concentric nucleus surrounded by a series of concentric arcs that resemble the cross-section of an onion or tree.— Sturman, N. The Microradiographic Structures of Non-Bead Cultured Pearls
The Mineralogical Museum of the University of Hamburg’s pearl testing service has an image of bead-nucleated cultured pearls under X-Ray, which shows the shell nucleus:
I found a lot more information at Gemological Institute America, which has this handy overview of pearls and this table on common treatments:
| Treatment | Purpose | Stability | Prevalence | Detection |
|---|---|---|---|---|
| Bleaching | Lightens Dark Spots; lightens some dark pearls to a brown ‘chocolate’ color | Stable | Routine for most types of light body color akoya cultured pearls; Occasional for other types of pearls | Some detectable by advanced testing by a gemological laboratory |
| Dyeing | Imparts or changes body color | Usually stable, but some colors may fade | Common | Some detectable by a trained gemologist, but often requires advanced testing by a gemological laboratory |
| Irradiation | Produces black, gray, or blue-gray color in white or cream-colored pearls | Stable | Occasional | Some detectable by a trained gemologist, but often requires advanced testing by a gemological laboratory |
The GIA also provides sample results of their Pearl Identification and Classification reports. Some research from the GIA:
- April ’09: Crystallographic Orientation of Biogenic Aragonite in Pearls (Run Lu) [pdf]
- May-Jun ’09: Melos and their Pearls in Vietnam (Vincent Pardieu) [pdf]
- June ’09: A Large Naturally Colored Natural Blister Pearl (Nick Sturman) [pdf]
- Aug ’09: The Microradiographic Structures of Non-Bead Cultured Pearls (N. Sturman) [pdf]
