What do you want to know? That's a pretty broad question
.
Diamond is crystallized carbon; sapphire is crystallized alumina, or aluminum oxide. Both are pretty hard, but diamond is much harder.
People tend to think of diamonds as colorless and sparkly; and sapphires as blue, and, well blue
. Truth is, both come in a rainbow variety of colors; moreover, a poorly-cut diamond can look lumpy, and a well-cut sapphire can sparkle nicely.
The nicest examples of both can be silly-expensive
, while both can be similarly affordable. At a particular size, diamond quality is determined more by how colorless and clean it is, whereas price for sapphire is determined more by just its color.
Diamonds are more typically untreated (though plenty of treatments occur), whereas sapphires are much more often treated (typically by heating) to improve color and clarity. I believe that a nicely-colored fine untreated sapphire is a much rarer thing than a nicely-colored fine untreated diamond.
Want more info? Start by taking a look at http://www.palagems.com/sapphire_connoisseurship.htm
Would the gemologists please correct and add to what I've said?