Asterism is a phenomena that occurs in gemstones which creates a star-like shape on the gem.
Asterism occurs when a gem has inclusions of rutile needles. The needles must be aligned in such a way as to reflect light in a multi-rayed pattern forming a star. The usual rule of thumb in gemstones is the fewer inclusions the better. However, in the case of asterism, the value is enhanced by desirable inclusions.
The value of a star gemstone depends on the clarity of the star. Generally the star must be evaluated using a single source of light such as a penlight; it is usually very difficult to see the star under diffused light. Moving the light back and forth should cause the star to move across the stone's surface. The rays of the star should be evenly distributed and relatively straight. The most common stars are 4-rayed and 6-rayed stars, though 12-rayed stars are not unheard of.
Colour is also very important in a star gemstone. Ideally the colour should be equivalent to a non-star specimen of the gem, but the presence of rutile in the stone tends to weaken the colour. So ideally, there should be enough rutile to create a distinct star whilst maintaining good colour.