Just a simple tip rather than a guide and please remember to do your research when purchasing stones if you are not trained in the field but simply love them like I do.
Saphires come in nearly all colours of the rainbow but the most well known colour is blue.
As a general rule the darker the australian saphire the less valuable it is - ie your 'black' saphire is likely to be purchased for about AU$25.00 per KILO!!! yet this is often marketed as though it is something particularly special.
Sometimes an australian blue sapphire can seem very dark but mayl still be good quality - to check see that it has good colour reflection from all angles and that when taken away from direct lighting that there is still a good deal of blue through the stone
The deeper the colour of a Ceylon Sapphire the greater the value is likely to be, the lighter and we have the ceylonese equivalent of the australian near black sapphire. A real good ceylon can be quite deep and rich in colour - similar in depth to what we novice buyers would sometimes expect to be an australian mid blue - if you come across a stone like this it could be a really great ceylon.
Along with the colour there is of course the clarity to be taken into consideration
Apparently the average price per carat of a very good quality sapphire is $600.00 so if you keep that in mind you may be able to bag yourself a real bargain in the auctions (this is as at June 2006)
Personally I like to try an buy for the colour I like rather than what may be the most valuable for that particular stone and I think that if we keep that as the main focus but have enough knowledge to try and protect ourselves so we don't get taken for a ride than we can't go too wrong. (unless someone misrepresents the stone they are selling)