01

What to look for

Under 10x loupe: extremely fine bright lines crossing in three directions at 60° angles. In transmitted light they appear as a subtle silvery shimmer. When very dense they create a milky haze inside the stone.

02

What it means

Intact silk confirms the stone is natural and has never been heat treated. Dissolved or broken silk means heat treatment has occurred, the most critical indicator in ruby and sapphire grading.

03

Expert move

Tilt the stone under a strong side light. The silk shimmers. Compare zones: treated stones may show dissolved silk on one side and intact silk on the other, revealing the treatment boundary.