Acorn
Silver-bordered cards are not tournament-legal and are not covered by the comprehensive rules.
Un-sets
So far the Un-sets (Unglued, Unhinged, and Unstable) are the exclusive place where silver-bordered cards were printed.
Mechanics
They feature mechanics that would be impossible to print in a normal expansion.[1] Silver border is also used for certain promos like the Holiday cards and the HASCON promos.
Rules
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