The singularity is that they are seen by people wide awake in glass balls and so forth.
Usually the seer is a person whose ordinary "mental imagery" is particularly vivid.

But every "visualiser" is not a crystal seer.

A novelist of my acquaintance can "visualise" so well that, having forgotten an address and lost the letter on which it was written, he called up a mental picture of the letter, and so discovered the address.

But this very popular writer can see no visions in a crystal ball.

Another very popular novelist can see them; little dramas are acted out in the ball for his edification.