Summons

[Masamune·1600]

You can also get the master summon materia from beating Ruby Weapon

Defeating Ruby Weapon yields the Desert Rose, which can be traded for a Gold Chocobo. The man in Kalm will give you the complete Master materia set for an Earth Harp, which is obtained after defeating Emerald Weapon.

I thought there was an upper limit to the number of materia orbs you could carry. But you won't hit the limit if you keep blending master materia.

The party can carry, at any one time, up to and including 200 non-allocated materia. If you gave every character, both active and inactive, eight-slot weapons and eight-slot armor, then you could potentially carry an additional 128 materia. Thus, the absolute ceiling for total materia is 328.