Trapiche emerald is a rare variety of the gemstone emerald, characterized by a six-pointed radial pattern of ray-like spokes of dark impurities

Emerald is a gem variety of beryl, a cyclosilicate with the ideal formula Be3Al2Si6O18. Its structure is characterized by six-membered rings of silica tetrahedra lying in planes parallel to (0001). 

The presence of trace amounts of the metal elements chromium & sometimes vanadium do not play particularly well together, and can cause minute stresses, creating the typical inclusions seen in most emeralds.

Emeralds are in fact a very hard stone (they sit at 7.5 – 8 on the Moh’s hardness scale

It is one of several types of trapiche or trapiche-like minerals, which also include trapiche ruby, sapphire, garnet, chiastolite and tourmaline. The name comes from the Spanish term trapiche, a sugar mill, because of the resemblance of the pattern to the spokes of a grinding wheel. 

During the formation of an emerald crystal, black carbon impurities may enter the gemstone mix. Because of emerald’s hexagonal crystal structure, these impurities may fill in at the crystal junctions, forming a six-point radial pattern.

In some trapiche emeralds, inclusions of albite, quartz, carbonaceous materials, or lutite may outline the hexagonal emerald core. From there, they extend in spokes that divide the surrounding emerald material into six trapezoidal sectors.

http://www.geologyin.com/2017/12/how-does-trapiche-emerald-form.html
https://evejewelry.wordpress.com/2013/05/07/romancing-the-stone-in-may/