gall stones are caused when you make too much bile and this causes it to precipitate in the gallbladder. This is when the solid stuff in them comes out of solution and forms little pebble shaped objects.
I think alot of the time it can be caused by hormonal changes such as during pregnancy. Being overweight can also increase the risk.
Gall stones can also be made out of cholesterol. i think these are slightly more common than the type Ollie mentioned. Whats interesting is that the cholesterol that forms these gall stones is not the same as the type in your blood.
they are more common in woment over 40 🙂 Thanks for your question