First, you need to find the volume of the metal.
V = 4/3 πr³ = 4/3 π(0.25)³
Density is equal to g/cm³ or kg/m³. Therefore, the density of the metal is
503 kg /[4π(0.25)³ /3] m³ <--Let this be equal to d
The specific gravity (SG) is the ratio of its density to water. The density of water is 1000kg/m³. <--I'll let this equal to w
So, SG = d/w <--just plug in the values you computed
Note: Sorry I don't have a calculator with me