Today, I learned new things about materia:

[Tint]

Also you can combine materia for a better materia (even different type of materia).

I want to add that when you combine the materia, you will never get the same type of materia out of it wich you used for combining.

For example when you used a crit+ materia for combining you will not get a crit+ materia as a result. So keep that in mind when you throw some lower grade materia into the mix. A crit V materia will make it impossible for you to get a crit VI out of it.