From Discworld MUD Wiki
Ambergris can be used to purchase special items sold in Niknax's shop.
a piece of white ambergris This is a piece of ambergris, a rare waxy mineral found on beaches that's prized for its sweet, musky smell. The paler the ambergris, the better its fragrance - this small marbled white lump is worth a great deal to the right people.
Ambergris is a continuous item, which means they stack and only count as one item even if you have lots.
Four pieces of white ambergris weighs 1 g.