Three-leaf clover necklace was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
A three-leaf clover necklace is an uncommon item won on the Squeal of Fortune. When worn, it increases gold coin drop amounts from monsters, and also increases experience across all skills by 2% while it has charges. For every charge consumed by training a skill for a period of time, it rolls for an item from the Squeal of Fortune, including lamps, cash bags, Pendants and lucky equipment. A newly won Three-leaf clover necklace has 10 charges.
While levelling a resource-gathering skill, or training combat, it randomly rewards gold or an item on the SOF drop table once every 10 minutes, using up one charge each time. It has a 10% chance of landing on the drop table, awarding one of the items listed among it. If an item (possibly a cash bag) is won, it will automatically appear in your inventory; the items cannot be converted to coins. If coins are won, they will go straight to your money pouch. If nothing is won, there is no message and the only evidence is that the necklace will show fewer charges remaining when examined.
Your three-leaf clover necklace shines briefly and you find <item>.