Sometimes an error slips through during production. Collector's often like these for the rarity.

Metallic Strip

Sometimes a bit of metal gets mixed in the glass and ends up on a marble

Bifurcated

Definition of bifurcated : divided into two branches or parts

Pieces of oven brick sometimes find their way into a marble

Another marble with a piece of firebrick inside

Peerless Patch with Metallic Mottling

Double stamped comic marble

Sometimes things got horribly wrong!