You are ready to commence your learning? Good. As you will have gathered by now, materia is the crystallized form of one's spiritual energy.
As this energy is derived from an individual's psyche─a volatile beast at the best of times─a degree of variance is to be expected in its production. In practice, this means that you won't know the exact properties of a stone until you cradle it in your hand.
Now, materia won't grant you much of anything on its own. Its power must be tapped, see, and this is achieved by melding it to gear. Know, though, that it takes the deft hands of a craftsman to meld materia.
You can always seek others to attach materia on your behalf, but if you would sooner do it yourself, I recommend you speak with F'hobhas. Melding is her
field of expertise.