Murky Matt is one of the current "Market Price Guides" near the east side of the Grand Exchange Market in north-west Varrock. He tells the player current prices of runes.
Murky Matt can combine players' enchanted jewellery charges, for a more efficient use, much like decanting. Two games necklaces with 1 charge each can be converted into one games necklace with 2 charges, for example. He can combine noted jewellery.
It is noted that if a player who has completed Lunar Diplomacy talks to him, Murky will refer to how "pirates" were stuck for days sailing around in circles. Apparently Matt is the brother of Murky Pat, a pirate currently working aboard the Lady Zay in the Lunar Sea.
If players wait long enough, they might see Murky Matt yelling phrases, such as, "Yarr, I'm gonna be rich!" or "Noo, All me prices be going down!"