marrow, core, kernel, nub, center, essence, meat, nitty-gritty, substance, inwardness, more...

periphery, outer layer, exterior

pithy, trivial, superficial, insubstantial, meaningless, tangential, obtuse, circuitous, more...

white, very, central, great, large, soft, elder, inner, whole, bitter, more

at the crux,   fruity pulp,   fruit core,   nerve of