I forego & skip the sidequest to get both the coal and diamond AND since I'm playing the game in the BGB emulator on my PC, I save & reload the game using only "save states" until I reach and beat Ganon; that is when I start to use the Save and Quit option. Only when I beat the game then I begin to get the coal and diamond items.
Do NOT use the "Save and Quit" option too much AND do NOT try to fill up the inventory with too many items (I mean do NOT buy the candle after beating the first two shrines as the candle item becomes useless after the 2nd shrine). that's when things go haywire.