[crystal-reports] Number to String in a formula field

I believe this is what you're looking for:

Convert Decimal Numbers to Text showing only the non-zero decimals

Especially this line might be helpful:

StringVar text     :=  Totext ( {Your.NumberField} , 6 , ""  )  ;

The first parameter is the decimal to be converted, the second parameter is the number of decimal places and the third parameter is the separator for thousands/millions etc.