Is there any type of fiber known to mankind that cannot be seen through with an X-Ray (or similar type) machine? If so, how tough or fragile would it
be?
I don't know about X-ray, but there are keramic pistols - you can go through a metal detector without any problems.
edit : of course there are also materials which canot be penentrated with X-rays but the operator will see that the beams are not capable to see
through so they will probably search you through.