its a combination of a training manual, virtual teacher, and a storage device, you can learn a trade without them, it just would take years if not decades to learn to the mastery one can with a crystal, the whole carved bit is in reference to the opening cinematic to fft
"a warrior takes sword in hand, clasping a gem to his heart.
engraving vanishing memories into the sword, he places finely honed skills into the stone.
spoken from the sword, handed down from the stone."
hence the reason we need both weapon AND stone to access the abilities within, and cant mix and match a spear with gladiator abilities despite the original dragoon in ff2 wielding a sword, and you see the creation of new 'blank' crystals through mch, the virtual teacher through a certain job, and newly formed 'job for dummies' crystals through blu. thats part of what makes the wol so powerful, is we possess not only the luck to gain many crystals rather quickly, but push our bodies and knowledge so hard that we are forced to learn new abilities and push our limits constantly
as for martyn, selling basically a riddle book to a bunch of people who are barely literate (not stupid, just only literate enough to function in daily life) probably wasnt the best way to go about spreading blue magic even if it was moderately more profitable to sell to the masses than to spend the time being picky about your clientele for more money per sale. that and most people that bought from martyn DIDNT automatically get hydro pump or whatever its called taught to them, they only saw martyn use it BEFORE they got their hands on a crystal... so technically the only attack most of them knew was pimp cane
