This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
No. In the story itself, there is Sephiroth's Black Materia that he uses
to cast Meteo, and Aeris' White Materia that is used to cast Holy, but
these materia aren't real, usable items. There are also seven materia
present that can't be obtained through normal means. In the US version,
they have been left untranslated, (and were for some reason altered in
terms of number of level stars, stats., etc.), but the Japanese version
of FF7 had fairly accurate descriptions of them. They are:
A Command materia (yellow). It has 2 levels. It lets you use Coin Toss
and Throw (unmentioned in the description).
A Combination materia (blue). It has 5 levels. It also has no
description, and apparently has no effect in terms of gameplay, either.
SuichuuKokyuu (Underwater Breath/Respiration)
An Independent materia (purple). It has 1 level. It also has no
description, and apparently had no effect in terms of gameplay in the
Japanese version. For the US release, this was made into the
'Underwater' materia that is used to nullify the time limit when
fighting the Emerald Weapon.
A Magic materia (green). It has 3 levels, and lets you cast the MBarrier
spell at each level.
A Magic materia (green). It has 4 levels, and lets you cast the Reflect
and Wall spells.
Rifuabu (no translation, but has something to do with 'reflect')
A Magic materia (green). It has 3 levels, and lets you cast the Wall
spell after it reaches 2nd level.
In the case of the three Magic materia, they would affect your stats as
so when equipped: Strength -02, Magic +02, MaxHP -05%, Vitality -01,
Magic def +01, MaxMP +05%.