What Huck means when he says that Mary Jane had grit and was full of sand?

Man ya'll useless. I'm completely oblivious & (and) feel like an ignoramus (an ignorant person) (my favorite substitute for the nig*** Word).

But my best guess is, gotta be, had "grit" means that she had the guts or courage to pray & "full O' sand" probably means (key Word : probably) that she was just going with the plan. You use & control sand, so, it must means that she was just following orders & the plan, nothing else.