Weve looked at the 'micro' perspective. Now lets look at the 'macro'.
Any form of alteration which leaves a product 'appearing' to be more valuable is IMO, a NO NO.
In the case of the marbelised cores, an existing core is merely being used as a substitute for any other 'base'. I dont consider that any different to making a unique (LE1 ) custom chip.
I would speculate that the intrinsic value of any such core after marbelisation is approximately 'zero'. The only perceived value is for the craftmanship that has gone into the new piece.