RED EMERALD CATSEYE CABOCHON
1.13 carat Teardrop Shape
 
All the colors of beryl can contain 'two-phase' inclusions, which means material in two phases of matter (usually a solid and gas) are present inside.  Both red and green carry a significantly higher concentration of inclusions, and this dramatically contributes to an Emerald's signature appearance.
 
Both red and green beryl display a very specific type of two-phase inclusion:  The tubular, fibrous or needle-like inclusion capable of transmitting or redirecting light.  Thanks to the repetitive manner of crystal growth, a specimen can 'line up' Two-Phase inclusions in the same way over and over in the molecular lattice, which results in those pieces being Directionally-Oriented (Photo 2, from Gems & Gemology, Winter 1984, by Robert Kane). 
 
When an army of these inclusions face the same direction, a stone can be shaped into a rounded dome with inclusions perpendicular to a gemstone's table.  Rare fibrous and wisp-like surface-reaching features can then gather, transmit and focus light...arranging their collected luminescence into a line.  Rocking this stone back and forth allows a viewer to see a line moving from one side of the table to the other.  This effect is known as the Catseye Phenomenon.
 
I have personally documented less than ten Red Emerald catseye examples, and this pear is the largest of them all.