Wow. What a beautiful piece!
You could try locating a mineralogist for a valuation. Where are you located? Perhaps you are near a university with a geology department, where you might get some help? An easy answer would be to simply take the carat weight of your piece and multiply that by the current value per carat for a nice piece of malachite. But this such a gorgeous piece, I think it would be hard to put a value on it.
Another possibility is a natural history museum. They sometimes have geologists or mineralogists on staff.
Good luck, LADYBUG