This is an interesting question. I play the cello so I have given this some thought earlier and I can´t help feeling that a cler crystal quartz would be very beneficial for in tune playing (-apart from practise, practise and more practise, of course
). I´m wondering if different instruments would benefit from different crystals, though? I mean in the same way that different tarot decks seem to like different crystals; my Vampires Tarot prefers rubies and my Undersea Tarot likes to play with Aquamarines.
Maybe the flute would benefit from higher frequency crystals (quartz) than the cello for example? I have actually been storing a small Jasper in my cello case. Initially just because it was the same color as my cello but it seems to suit it well. Maybe this is a thing one has to just intuit? Maybe no-one knows but you and your flute?