Grum is a man who runs Grum's Gold Exchange in Port Sarim. He buys and sells unenchanted rings, amulets and necklaces from sapphire up to diamond.
Grum's clothes are various shades of purple. Since purple dye was expensive in medieval Europe, Grum may wear this colour to reflect his wealth. Being a jewellery trader, this would make sense.
During Merlin's Crystal quest, players will have to bring a loaf of bread to his gold exchange and give it to a beggar in order to receive the Excalibur from The Lady of the Lake.