All Credits to Hoppy the Hopper for the calc.[]
The rock is roughly a sphere, making its volume is about (4/3)*(10/2)^3*pi = 523.596 m^3
Skull Island is based on Ha Long Bay, so I will use limestone. So d=2300 kg/m^3 (must use low-end, since it isn't a perfect sphere)
So m= V*d = 523.596*2300 = 1204270.8 kg, or about 1204 tons
Extra juicy: Star Wars vs Star Trek: Asteroid Destruction Calculator
Slap 10m for diameter in and you get about 1 ton of tnt worth of fragmentation energy
Edit: Lower-end: The rock is a cube
Assuming 10 is the diagonal line of a face of the cube, then each side of the cube is 10/sprt2 => volume becomes 500/sprt2 or about 353.55 m3 => m = ~815 tons