I am working on singing better for my musical coming up (turns out I got a part where I have to sing very high so everyone can hear me) if there any crystal that I can use to help my diaphragm and therefor improve my singing (while I practice of course).
I get the sense that Harmotome and Rhodolite would be helpful for you.
Love is In the Earth says that harmotome is "useful to one who is actively engaged in musical pursuits".
Rhodolite is a type of garnet that helps "support the movement of... energy through the chakras". The diaphragm sits between the 3rd & 4th chakras and is a place where energy trying to move between the two can get stuck.
It helps the physical and subtle bodies stay in sync, and is "known as a 'stone of inspiration'"... while that's talked about in terms of the inspiration of love, inspiration is also another word for breathing, which is probably why this stone is said to also treat the lungs.
If you can't find rhodolite, I think any garnet could help.