More info: http://www.gia.edu/ruby-quality-factor
People in the trade expect rubies to have at least some inclusions because inclusion-free rubies are practically nonexistent. Ruby value depends on how visible the inclusions are. Obvious inclusions or inclusions that reduce transparency or brightness lower a ruby’s value dramatically. If large and prominent inclusions are located under the table facet, they greatly diminish the transparency, brilliance, and value of the stone. Inclusions can also limit a ruby’s durability. Significant surface-reaching fractures can pose durability threats.
Typical ruby clarity characteristics include thin mineral inclusions called needles. When the mineral is rutile and needles are present in intersecting groups, it is called silk. Needles might be short or long and slender, and they might appear to be woven tightly together.
Ruby can also contain needles composed of other minerals, small crystals, zones of color variation, or inclusions that resemble fingerprints.
Some inclusions can actually contribute positively to a gem’s appearance. The presence of rutile silk causes light to scatter across facets that might otherwise be too dark. This adds softness to the color and spreads the color more evenly across the ruby’s crown.
Needles that intersect can also cause the star effect, called asterism, when the stone is cut with a curved upper surface.
Several factors affect the cut and proportion of rubies on the market. A ruby’s crystal shape dictates its suitability for certain cuts. The most common shape is a flat tabular hexagonal shape, but ruby crystals from some sources can be elongated.
To accommodate these crystal shapes, the most common shapes of fashioned rubies are ovals and cushions, with brilliant-cut crowns of kite-shaped and triangular facets, and step-cut pavilions with concentric rows of rectangular or square facets.
Round, triangular, emerald-cut, pear, and marquise rubies are also available. But these shapes are rare in larger sizes and higher qualities.
Ruby rough is very expensive, so cutters try to conserve as much weight as possible. They might fashion flattened ruby rough into shallow stones, even though light escapes through flattened pavilions, causing an unattractive see-through area in the stone called a window.
Pleochroism—the appearance of different colors in different crystal directions—is another factor that influences cut. In ruby it typically appears as red to purplish red in one crystal direction and orangy red in the other. Cutters can minimize the orangy red color by orienting the table facet perpendicular to the long crystal direction. Even so, it’s not always possible to orient a ruby for ideal color return because the potential loss of weight would be too great.