Depending on the substance, you can use organic solvents, acids, melting with fluxes and dissolution, dissolution under high pressure and temperature, etc.
Usually, it does dissolve, but when it doesn't, I can come up with two reasons: 1. The water is too cold since an increased temperature increases the solubility. 2. The water is already saturated...
Whil oil is slightly soluble in water, it is a nonpolar molecule while water is extremely polar. This means there is very little attraction between them and it is only sparingly soluble.
The metal-metal bonds holding the gold together are relatively strong, which is why the melting point of gold is over 1000 oC. Now of course ionic compounds have melting points in excess of 1000 oC...