But why????i mean its a solid thing...so it should appear on xray right?
Not all solid objects appear on xray. Some objects are radiolucent meaning that they are translucent on xrays and other objects are radiopaque meaning they are visible to xray. Even though it might be a fake diamond it still might show up on xray with digital machines. You can see soooo much detail in digital xray... even peoples hair.