Sapphire sometimes displays a three-ray, six-point star.  These star sapphires are cut in a smooth domed cabochon cut to display the effect.  The star is best visible when illuminated with a single light source: it moves across the stone as the light moves. This effect, called asterism, is caused by light reflecting off tiny rutile needles, called "silk," which are oriented along the crystal faces.
The value of star sapphires are influenced by at least these two things: 1) the intensity and attractiveness of the body color, and  2) the strength and sharpness of the star.  Of course all six legs should be straight and equally prominent.  Star sapphires rarely have the combination of a fine translucent or transparent color and a sharp prominent star, but when offered, these gems are highly valued and the most expensive.
If you own one and aren't sure if it's genuine, here's a few quick ways to tell the difference... First thing to look at is the bottom, if there is an "L" stamped in the stone, it's a Lindy Star and synthetic... if it passes that test  look for imperfections within the stone, and/or unevenness on the bottom, and/or stripes or lines of color that shows through the top, most naturals have one or more of these natural imperfections... then look at the star itself, using a flashlight, most natural stars do not have a "perfect" star, whereas the natural will most likely have 1 or more of the 6 legs not exactly the same length, or maybe not all 6 are perfectly straight... also the star must travel around following the light source, whereas if the star is stationary it is definitely synthetic, the most common synthetics jump out as way are too perfect looking.  You can also take it to a jeweler that has a diamond tester (with has a needle gauge instead of the ones that just making a beep), the dial should jump to the mark just before a diamond to tell you it's corundum.  These are just simple things you can do to rule out the majority of synthetics, but if you have a perfect star, a super clean stone with great color, the only way to be sure is to send the stone to a reputable Gem Laboratory and ask for a Gem Identification Report.  Prices for this report will range between $100 to $500, depending on how much detail you want on the report.  If you don't know who to contact, here are a few links to Accredited Gem Labs: AGA | AGL | AGS | AGTA | CCGL | EGL | GIA | GQI | GRS | GUBELIN | GUILD | IGI | LPSL | SSEF
TREATMENTS - Star Sapphire is normally "not treated", since the treatment process would dissolve the rutile needles that causes the asterism to form a star.
CARE - It is usually safe to clean Star Sapphire in an Ultrasonic cleaner, but risky to use a steamer. We recommend ionic cleaners and/or warm, soapy water and a soft brush as the best way to clean Sapphire jewelry.
Our source for fine Star Sapphire is Sri Lanka (formerly Ceylon), known for producing the finest variety of Sapphire available.  Star Sapphires are highly valued Collectors Gemstones.