Simple solution to this is make self melding available with alexandrite (just like the relic was melded) and have the alexandrite able to be bought with X number of poetics or esos. Same rules apply as relic. Breaking materia doesn't break the alexandrite.

Crafters can craft and sell the alexandrite. Best of both worlds much like glamour prisms. If you are acrafter you can make them, if you are not you buy them from a crafter.

I do not think you should be able to just meld something without some type of prerequisite catalyst (alex) or qualifier (crafting level).