Since they're made partly of calcium, they'll usually be radiopaque, meaning you can see them on x-ray, here's an example showing pigmented gallstones on x-ray.
Sometimes, instead of oxalate, the calcium binds a negatively charged phosphate group to form calcium phosphate stones, which are dirty white in color and alsop radiopaque on an Xray.
A tiny minority of stones are cystine stones, composed of the amino acid cystine which sometimes leaks into the urine to crystalize and form a yellow or light pink colored stone that is radiopaque under X-ray.