Genuine "star" sapphires and rubies are rare and valuable. The "star" pattern in the stone is created by crystal growth lines.

"Star" stones have an opaque rather than a transparent finish, for better light reflection. The back (or bottom part) of the stone remains unpolished, so there is no light leakage. The round (cabochon shape) top is necessary to bring out the star. Fine stars are pretty rare. Until the 1960s, these stones always sold for more than the faceted ruby.
A recent introduction in the market are diffusion star sapphires, which are of the same material as natural sapphires. They don't contain a good colour and/or a star naturally, so by the addition of chemicals and/or heat colour a star is added artificially to the surface layer of the stone.
Legends tell us that the three intersecting rays are thought of to represent faith, hope and destiny. Ancient cultures have thought that star sapphires had demons or angels of light living in them, and because they turned dark at night, they were thought to go to sleep.

The best stars will have just enough silk to create the star effect, but not so much as to harm the transparency and colour. Natural stars are normally non-heated stones. The rutile silk is what actually forms the star. If a stone is heat-treated, then you lose its star because the rutile needles are destroyed when heated.
If a fine star ruby or sapphire is properly lit, the needles reflect the lighting in such a manner that it appears the star is hovering over the gemstone. The star wanders as the light is moved.

How rare are these gems? For every 100 faceted corundum (ruby and sapphire) mined, approximately three stars are discovered. One will have good colour and a bad star. One will have a great star and bad colour. Only one out of a hundred will have a good star and good colour.