Broken plate was discontinued from RuneScape after an update, but still exists in-game.

Broken plate

Release date	4 April 2005 (Update)
Members?	No
Quest item?	No
Tradeable?	No
Equipable?	No
Stackable?	No
High Alch	0 coins
Low Alch	0 coins
Value	0 coins
Destroy	Drop
Store price	Not sold
Examine	Alas...
Weight	0 kg

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The broken plate was the item one could receive by spinning a spinning plate shortly after the 2005 April Fools event. There was a chance you might drop it, and it would break into this. Due to player complaints, Jagex removed the broken plate, and plates now cannot be broken.

However, Jagex never removed the already broken plates from the game, though they are untradeable, they are discontinued. It is unknown how many of these plates are left in RuneScape. Since they cannot be traded, a price cannot be put on them, thus leaving their rarity in question. Some speculate they are rarer than a party hat, being as they cannot be traded, and therefore were dropped by many.