Do all kidney stones show on a ct?

Almost all do: 99% of stones are radiopaque and therefore visualized by computed tomography. There are rare renal stones (i.e. Indinavir) that usually do not show up on ct.

Usually: Both calcium and uric acid stones show on ct.

No: Most stones are visible on ct but there are very few that may not be seen.

Yes: Yes, a non-contrast ct scan is very sensitive for even tiny stones. Unlike plain x-ray, even non-calcific uric acid stones generally can be identified.