HI Navi and Spiritlite
Interesting........Ok, my intuiting 
feels that the crystals would prefer to be
held nearby or laid next to the bowl rather
than in the bowl. Recall Crystals & stones
interact with our energy fields and thus
that of the Singing Bowls within three feet!
It may be deleterious to it in the bowl is
my sensing...or no prob?! I would definitely ask
the crystal for permission first for a procedure
like that... 
That is my view with the crystals...
I am a cautious Capricorn most of the time~ Thanks
for your thoughts & posts here to each of you!
Since you already did some, Navi then I suppose
for those crystals it was okay fine. 
Spiritlite
your rose quartz feels like the Bowl's assistant!
Love ~ 
~ Crystalline*Light,
Volet~Maya