Convert a Double to Hex notation in Swift

How do I convert a very large number into hex?

For example, 647751843213568900000 in hex is 0x231d5cd577654ceab3. I'm able to easily go from hex to double with:

let hex: Double = 0x231d5cd577654ceab3

However I can't work out how to go from Double back to hex. What am I missing?

The following does not work as it overflows when stored as an 'Int':

let hexConverted = String(647751843213568900000, radix: 16)