The universal currency in Kongo and just about all of Central Africa was the shell of "Olivella nana", a sea snail, known locally as "nzimbu". One hundred nzimbu could purchase a hen; 300 a garden hoe and 2000 a goat. Slaves, which were always a part of Kongo's economy but increased in trade after contact with Portugal were also bought in nzimbu. A female slave could be purchased (or sold) for 20,000 nzimbu and male slave for 30,000. Nzimbu shells were collected from the island of Luanda and kept as a royal monopoly. The smaller shells were filtered out so that only the large shells entered the marketplace as currency. The Kongo would not trade for gold or silver, but nzimbu shells, often put in pots in special increments, could buy anything. Kongo's "money pots" held increments of 40, 100, 250, 400, and 500. For especially large purchases, there were standardized units such as a funda (1,000 big shells), Lufuku (10,000 big shells) and a kofo (20,000 big shells).