Silk

Silk is the trade's word for fine needle-like inclusions, usually rutile, laid along the crystal directions of corundum. In sapphire and ruby the needles run in three directions in one plane, meeting at sixty degrees, and under magnification they look like woven fabric, hence the name. One feature, read three ways. Intact silk argues against high-temperature heating, since heat dissolves it. Dissolved silk, seen as broken or dotted remnants, argues for it. And finely dispersed silk scatters light into the soft glow the trade prizes in Kashmir sapphire and in the best of Mogok, the velvet that makes a photograph fail. Dense oriented silk, cut en cabochon, is what makes the star.

So the same needles that lower clarity can raise value. A Kashmir sapphire with no silk at all would raise more eyebrows than one with plenty.

On a lab report

Silk seldom appears as a word on the face of a report. It appears as evidence. When GIA writes "no indications of heating", or SSEF writes the same phrase in its comments, part of what the gemmologist has seen is silk in its original state; when a stone comes back "indications of heating", altered silk is often the reason. Lotus, Gübelin and AIGS attach photomicrographs, and the silk is usually in them. The line is not absolute. Low-temperature heating, common in Mozambique ruby, can leave much of the silk intact, so laboratories lean on other tests as well.

Ask to see the stone under a 10x lens, in daylight, and ask the dealer where the silk is. Fine, evenly spread silk in a sapphire is a feature. A dense clot in one corner is a clarity problem. And any silk at all in a stone sold as heated deserves a second look at the report, because the two facts do not always sit together.