Under darkfield illumination at 30× magnification, a gemmologist examines a red corundum cabochon. They observe extremely fine, bright, needle-like inclusions arranged in three intersecting directions, meeting at approximately 60° angles throughout the stone. The needles are short and appear slightly interrupted in places, as if broken into dotted segments. The stone has been described by the seller as "untreated Burmese ruby." What do the broken, dotted needles indicate about the stone's treatment history?

The interrupted (dotted) needles indicate partial heat treatment; intact silk suggests unheated, fully dissolved silk suggests high-heat treatment, and broken or dotted silk is diagnostic of moderate-heat treatment that partially dissolved the rutile needles

A student examines two blue sapphire cabochons from different origins under the microscope. Stone A (described as Sri Lankan/Ceylon) shows long, prominent, clearly separated rutile needles extending across the full field of view and intersecting at 60°. Stone B (described as Burmese, Mogok) shows very fine, short, densely packed needles that give the stone a soft, slightly hazy appearance. Which description matches which origin, and what trade term is used for Stone B's appearance?

Stone A is Sri Lankan (long, prominent needles); Stone B is Burmese Mogok (fine, dense, short needles producing the 'velvety' or 'sleepy' appearance valued in fine Burmese ruby and sapphire)

gemmology.dev – Silk & Needle Inclusions

Introduction

Silk refers to fine, needle-like inclusions that create a silky lustre in gemstones.
Most commonly composed of rutile (TiO2) in corundum, silk is one of the most
important diagnostic features for both identification and origin determination.

Rutile Silk in Corundum

Rutile silk is the defining internal feature of many natural corundums ^[1].
TiO₂ crystals exsolved from the corundum lattice during cooling, oriented at 60° intersections:

Composition:

Titanium dioxide (TiO2) crystals
Exsolved from corundum lattice during cooling
Oriented along crystallographic directions (60° intersections)

Appearance:

Fine, needle-like crystals
Often intersecting at 60° angles
Creates silky sheen when dense
Length and density vary by origin

Silk and Geographic Origin

The characteristics of rutile silk are crucial for origin determination in corundum.
Short, fine needles indicate Burmese origin; longer needles at 60° indicate Sri Lankan origin
^[2].

Burmese Corundum

Short, fine rutile needles {cite:hughes-ruby-sapphire}
Delicate, wispy appearance
Creates soft, velvety look
"Finest" silk considered

Sri Lankan (Ceylon)

Longer needles at 60° {cite:hughes-ruby-sapphire}
Clearly visible individual needles
Often creates strong asterism
More widely spaced

Thai/Cambodian

Dense networks of silk
Often very heavy coverage
Can make stones appear cloudy
Iron-rich environment

Kashmir Sapphire Silk

Silk and Asterism

Dense, properly oriented silk creates star effects (asterism). Three sets of rutile
needles at 60° create a six-ray star ^[1]:

Requirements for asterism:

Sufficient density of silk needles
Proper orientation (intersecting at 60°)
Correct cutting (cabochon, oriented perpendicular to c-axis)

Star types:

6-ray star - Three sets of silk at 60° (common in corundum)
12-ray star - Six sets of needles (rare, exceptional)
4-ray star - Two sets at 90° (in garnets, diopside)

Heat Treatment Effects on Silk

Heat treatment significantly affects silk appearance:

Low to moderate heat:

Silk may partially dissolve
Needles become interrupted ("dotted silk")
Stone clarity improves

High heat:

Silk completely dissolves
Titanium re-enters corundum lattice
Stone becomes cleaner but loses silk

Diagnostic value:

Intact silk suggests no high-temperature treatment
Dissolved/dotted silk indicates heat treatment ^[1] ^[3]
Complete absence may indicate treatment or origin

Other Needle Inclusions

Tourmaline Trichites

Tourmaline contains distinctive needle-like inclusions:

Trichites: Hair-like thread inclusions
Often in parallel groups
Different from rutile silk
Characteristic of species

Amphibole Needles

Found in various gemstones:

In emerald: Tremolite/actinolite needles (especially Afghan, Russian)
In demantoid: Byssolite (actinolite) "horsetail"
In peridot: Ludwigite needles (rare)

Garnet Silk

Some garnets contain silk-like inclusions:

Almandine often has rutile needles
Rhodolite may show fine silk
Creates asterism in some star garnets

Needle Inclusions by Gemstone

Gemstone	Needle Type	Characteristics
Corundum	Rutile silk	60° intersections, origin-diagnostic
Tourmaline	Trichites, growth tubes	Parallel to c-axis
Demantoid	Byssolite (actinolite)	Horsetail pattern
Almandine garnet	Rutile	Can create 4-ray stars
Peridot	Ludwigite	Brown needles, rare
Diopside	Magnetite	Creates 4-ray stars

Silk & Needle Inclusions