[Rutile needles (silk) aligned in three directions in unheated corundum]
[Star ruby, white star sapphire ring, rare bi-colored star sapphire] ...
The rutile needles tend to align with the c axis of the host quartz, and can run the entire length of the crystal.
Small rutile needles present in gems are responsible for an optical phenomenon known as asterism. Asterated gems are known as "star" gems.
Included rutile needles or hematite plates are called 'silk', which bring about a soft sheen in the stone.
Star sapphire is the result of reflection of light from fine, oriented, rutile needles. Like star ruby, star sapphires may be heat treated to high temperatures to dissolve the rutile and produce blue sapphires of good clarity.
To help identify an unheated Burma fancy sapphire, try to locate short rutile needles that intersect at 60 degree angles and inclusions that are intact (unexploded).
If there are no rutile needles within a cabochon sapphire you will only see color and not the phenomenon of a star. Stars have been used in jewelry for as long as sapphires have been cut.
"Silk," networks of fine, included rutile needles, intersecting at 60 degree angles, are commonly seen in natural ruby
Included crystals of zircon, and related stress fractures (or halos) are sometimes seen.
Unheated Burmese stones may contain "silk"; silk is very short fine rutile needles that form at 60 degree angles to each other. When heated the silk burns out or alters, so silk can be positive value factor. Unheated rubies command a premium price.
Sapphire often contains minor inclusions of tiny, slender Rutile needles. When present, these inclusions decrease the transparency of a stone and are known as silk.
The variety of chrysoberyl containing rutile needles or negative crystals oriented in the same direction showing some exceptional and sharp chatoyancy is called cat's eye.
Sapphires with inclusions of tiny, rutile needles exhibit an optical property called asterism. This is the star shaped effect seen in star sapphires and is usually only seen in cabochon cuts.
This effect, called asterism, is caused by light reflecting off tiny rutile needles, called "silk," which are oriented along the crystal faces.
Also found with the rutile needles making a cats' eye or marvelous 6 rayed star when cut en cabochon. Most popular as a blue stone and the most desired are Kashmir sapphires, although they now come from Burma.
Inclusions such as these rutile needles in ruby are often strikingly beautiful. Gemoloigsts study inclusions for clues about the origin and nature of gems.
The rutile needles can be reddish (which is what rutile means in Latin). Often found embedded in quartz; can also produce some amazing gem phenomena as a star and a cat's eye in sapphire and other gems.
These inclusions may appear as clouds, feathers, veils, silk fibres or rutile needles.
To further understand external beauty, we must delve internally to see the features that define the genesis of spinel: randomly oriented growth tubes, rutile needles in an obscure phantom pattern, and platelets flashing spectral colors.
Mogok rubies tend to be well formed, rounded rubies that may contain patches, zoning or overall cloudiness created by short, fine rutile needles or silk inclusions.
inclusions (n) are materials that are trapped within a mineral during its formation. Inclusions range from rutile needles, insects, rocks, water and gas.
iridescence (n) ...
A luminous star like effect exhibited in some gemstones like Star Sapphires, Garnets and Rubies. Asterism is caused by inclusions of tiny, parallel, rutile needles and may result in four, six or even twelve rayed stars. (Pronounced: as-ter-iz-mm) ...
Other minerals include microcline and orthoclase feldspars, gray quartz, red-violet garnets and rutile needles.
transparent to opaque brownish to red to black mineral of a higher refractive index than diamond that occurs in a variety of minerals. It is accepted as an important acicular inclusions in many gemstones. When quartz contain numerous rutile needles ...
Summer 1976, Included rutile needles in emerald, p. 174, 1p.
Fall 1976, A quartz crystal inclusion in Colombian emerald, p. 213, 1p. (See also Summer 1977, p. 314, 1p.)
Fall 1977, Very strong color zoning in emerald, p. 328, 2pp.
 See also: Needle, Rutile, Stone, Color, Inclusion
  
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