Hi Axelg,
I have used crystals when I do reiki and the feedback I've had is that reiki is definitely more powerful with crystals. I use my diviner and hold over a chakra and ask which crystal or crystals are needed for that chakra and then divine over my crystals to find out which ones are needed. I give the person laying on the bed a crystal point in each hand, in their left hand point it towards the head and in their right hand point it down to the feet so it helps focus the energy to go round the body. People definitely feel better when they have had a 'reiki with crystals' session with me.
Love and light
Landy