If anything your using quartz will enhance your reiki concentration. Many reiki-ites use crystals with their reiki as crystal grids and as channellers and distance healing methods.
Did you know that ther are crystal reiki grids placed at each pole?
This is the North Pole Grid
This is the South Pole Grid
The Amethyst.. its a proven scientific fact that the manganese in Amethyst does break down in UV light, so although our crystals look much happier ( and I am sure they are after a lifetime underground lol) they will slowly change in colour. Personally I don't mind it at all as some beautiful lavender colour can come through

Being on a shelf isnt necessarily the answer, the darkening can be from electromagnetic forces, is it near a pc or tv or anything electrical? I still keep some of my Amethyst in the sun , happy and beautiful
Hmm I feel the need for some pics lol.......

My Large Amey cathedral

My Amethyst wishing well