Silk is a mass of fine, crossing needle inclusions, usually rutile, in ruby and sapphire.
In more detail
A little silk scatters light and spreads color over dark areas, which can soften and even out the color. Too much silk lowers transparency and value. Intact silk shows that a stone was not heated to very high temperatures, but not that it was never heated.
On a grading report
Not listed as a grade. Its state helps a lab decide between 'no indications of heating' and 'indications of heating'.
Related terms
- Needle: A needle is a thin, elongated crystal inclusion that looks like a tiny rod at 10x.
- Asterism: Asterism is a star of light that glides across the dome of a gem. Light reflects from fine needles, often rutile, that lie in two or three directions.
- No indications of heating: 'No indications of heating' (NH) is the standard lab wording for a corundum that shows no signs of heat treatment.
- Heat treatment: Heat treatment is controlled heating of a gem to change its color or clarity.
- Ruby: Ruby is the red variety of the mineral corundum. Chromium causes its red color.