A fingerprint is a pattern of tiny fluid or glassy drops left where a crack inside the crystal healed. It often looks like a human fingerprint.
In more detail
Fingerprints are common in natural ruby and sapphire. Heating can heal fissures and leave glassy residues, which labs report. Flux-grown synthetics can also have flux fingerprints that look natural, so an expert must judge them.
On a grading report
After heating, labs may describe residues in healed fissures as minor, moderate or significant.
Related terms
- Residues in healed fissures: Residues in healed fissures are remains of the heating process, often glassy. They stay in fissures that healed while a corundum was heated.
- Flux growth: Flux growth is a way to grow synthetic crystals from a chemical solvent (flux). Crystals form as the hot solution slowly cools.
- Fissure: A fissure is a crack in a gem, often open to the surface. Labs use the word for the cracks that fillers can enter.
- Inclusion: An inclusion is a clarity characteristic inside a diamond, or one that reaches the surface from inside.
- Heat treatment: Heat treatment is controlled heating of a gem to change its color or clarity.