Cowries are marine gastropod molluscs of the family Cypraeidae. There are many varieties and although I can’t speak directly of their use I can give information about how shell money was used until modern times in New Britain. The money used there was from the Nassa Callosa shell, marine gastropod molluscs of the family Nassariidae. The main difference between these and Cowrie shells, although in the same size range, is that these have a pointed spiral tip.

The book; VUNAMAMI Economic Transformation in a Traditional Society, by Richard F. Salisbury Pub. 1970 by Melbourne University Press provides a detailed study of the use of shell money (called tabu) finance, and values from the 19th – 20th centuries. i.e. Historically it relies on European missionaries and explorers, and the current field work (1960s) of the writer. Tabu was still in use in the 1960s concurrently with cash money although not fully interchangeably as tabu had political, organizational and entrepreneurial uses among the Tolai people not fully available from cash and cash was needed for any dealings with the government or for imported goods.

In answer to your question about picking some up from the beach to enhance your wealth, sort of, but not casually. Although the nassa callosa shell is reasonably common and can occasionally be picked up casually, the shellfish grow in colonies off particular beaches, the individual shells aren’t used as money. Normally an expedition of between four and twenty men travel by sea from a village to a beach known to have colonies. They have to pay goods to whatever village is located there (there are seven villages with suitable beaches) in order to stay at the village and spend time harvesting the shell fish and may be there several months. Sometimes villages harvest themselves and take the shellfish to main towns for trade.
Once sufficient shells are collected the expedition returns home but the work has just begun. The lips of the shell have to be cut away from the whorl (the pointed spiral tip) and the lips are strung on lengths of rattan. This is women’s work and the women have to be paid for the work. It is these strings of shells that are used for money. A six inch string is the basic unit and is called a “pidik” (about 24 shells), six pidiks is called a “peapar”. Salisbury refers to twelve pidiks as a “fathom” although this is an English measurement meaning “six feet” but he counts large amounts of tabu as fathoms and he estimates a total of 1,730,000 fathoms of tabu as being in circulation in the 1960s. There are variations to the names of the lengths.

A pidik is essentially small change. Adults typically carry several with them and in 1881 six pidiks would buy a standard fish (which might be several smaller fish). There were different fixed prices in the market for different foodstuffs. By 1961 it took 12 pidiks to buy a standard fish so over eighty years inflation had halved the buying power, a very slow rate of inflation. The Tolai were aware of price changes and were concerned to maintain buying power in a set of fixed relationships between the costs of different foodstuffs. They particularly worried about the export of food for cash money pushing up prices. Salisbury noted that inflation was controlled because the expeditions to collect the shell had alternate economic uses for their time. At times when shell money buying power was low fewer expeditions would go out and those mostly formed of men with low economic power such as some old men or very young men.

Apart from small change tabu could also be used for large expenditures and was necessary for the accumulation of power and status within a community in ways that could not be achieved with cash. The lengths can be extended and stored within a household in skeins of several fathoms and very large amounts are stored as continuous lengths of rattan coiled and wrapped in leaves, the coil appearing like an automobile tyre and containing 200 to 300 fathoms. Larger coils contain 1,000 to 1,200 fathoms. Coils may belong to individuals, corporate groups (like villages) or religious ancestral groups.

Generally a coil is a display of wealth and a way of storing shell money not needed for immediate use, although it is expected that at some time the coil will be broken up and distributed. This may be for payment for a ceremony or festival where the person or group holding the ceremony will use it to pay for the food and to pay the participants. Ceremonial dancers need to be paid in tabu and the more senior the dancer (the more ceremonial dances they are qualified to perform) the more they need to be paid. This is one way of gaining large amounts of tabu. Holding ceremonies is expensive and gives status and political power to the “big man” or organization conducting them.

The largest and oldest coils are held by ancestral religious groups (funding ceremonies like funerals) and by holding large amounts of tabu for long periods they are effectively slowly withdrawing that shell money from current circulation which would also tend to slow inflation. Salisbury noted that he saw privately owned rolls up to twenty five years old but religious ones were typically older.

In Brief: Yes shells were used as money until quite recent times. You could collect shells yourself to increase your wealth but it would take a lot of work and some investment of goods. You would then need to have them transformed into money. The financial system using shell money does have controls built into it to reduce inflation.