Tales of Soft Money — The Sahelian Kingdoms
Some interesting findings on money in pre-colonial West Africa are described and summarized by Laurence Garenne-Marot, whose work is now published by the British Museum for anyone to read. He uses translated writings of ancient scholars as well as specific archaeological evidence to piece together parts of the complex value protocols of the Sahelian kingdoms, which more specifically means the pre-medieval Ghana Empire and it's geographical successor, the Mali Empire. His short essay starts off by quoting a person named Ibn Battúta.
Ibn Battúta was a 14th century Muslim scholar from Morocco who traveled far and wide over the medieval world. After a long journey that took him to the Arabian peninsula and then down the east coast of Africa, he proceeded to travel east through Asia until he reached China, after which he returned to his homeland. Instead of immediately continuing on further planned expeditions, he was sent on a slight detour to the Muslim-controlled part of southern Spain in anticipation of a Castilian attack (something that was postponed by the Christian kingdom after the plague took the life of it's ruler). Battúta promptly returned to Morocco where he prepared for his next travels: to the Muslim kingdoms of West Africa. A long, arduous journey on thousand year old trade routes slowly took him and his companions through the hot Saharan desert. The caravan then followed a river Ibn Battúta wrongly assumed to be the Nile, and soon enough they reached the capital of Mali where they could finally rest. Leaving for Timbuktu, he then continued east to Goa, and then further in to what today is Niger, and that is where this article first intertwines with the concept of easy and hard money. Upon arriving to the city of Takadda in 1352, Battúta recorded:
The copper mine is in the outskirts of Tagaddá. They dig the ore out of the ground, bring it to the town, and cast it in their houses. This work is done by their male and female slaves. When they obtain the red copper, they make it into bars a span and a half in length, some thin and others thick. The thick bars are sold at the rate of four hundred for a mithqál of gold, and the thin at the rate of six or seven hundred to the mithqál. They serve also as their medium of exchange; with the thick, slaves, male and female, millet, butter, and wheat. 
In other words, he had stumbled upon a copper based monetary system, with standardization efforts in terms of divisibility and weight (mithqál) of the minted copper bars (or rods). Battuta also mentioned that these copper bars were then "carried to different lands and among these to the lands of infidels".
Another Arabic writer is then quoted by Laurence Garenne-Marot in order to corroborate the authenticity of Battúta's copper bar money observations. The writer's name was al-'Umari, and he documented his interviews of Egyptian officials that had met Mansa Musa, a 14th century Emperor of Mali. Mansa Musa had allegedly told the Egyptians:
We send it [the copper] to the land of the pagan Sudan and sell it for two -thirds of its weight in gold, so that we sell 100 mithqáls of this copper for 66 2/3 mithqáls of gold. 
Boasting about this to the Egyptians was probably a bad idea in the long run as they, or others, as well could obtain copper and then try to barter with the copper-poor but gold-rich 'pagans of the forest'.
In addition to the documentations of Battúta and al-'Umari, a third glimpse into the West African value protocols of that time could be uncovered with the help of archaeology; Frenchmen R. Mauny and P. Thomassey, in 1949-1951, dug up standardized copper bars in the old Muslim merchant quarter of Kumbi Saleh - a town that was previously the capital of the vanished Ghana Empire and that had conducted a lucrative trans-Saharan trade with the Berbers and Arabs in the north. These copper bars were likely functioning as money, and were much older than those observed by Battúta further east. One interesting observation of the archaeological findings was that some of the copper bars had been diluted with lead, which meant they lost much of their workable use as consumable items. The implication is that as money, it didn't matter as much that the metal could not be re-smelted and consumed for non-monetary purposes.
Northwest of Kumbi Saleh lay the town of Tegdaoust - another end point of one of the many north-south trans-Saharan trade routes. New excavations during the 1960's and 70's gave a pretty clear view that copper money production occurred in the town's workshops at a time prior to Battúta's Mali expedition. Not only did these workshops smelt local copper ore to trade the metal with gold in the more copper-sparse parts outside the kingdom; the merchants and metal workers of the city were smart enough to likely import relatively cheap copper and brass from north of Sahara. Some evidence of this could be seen in the finding of a large number of copper bars buried in a sand dune in the Majâbat al-Koubrâ, the most arid part of the Mauritanian Sahara. That excavation point was on a trade route to Tegdaoust, and as the bars consisted not of unalloyed copper, but of brass (copper with high zinc content), it became clear that they were produced outside of Ghana/Mali (probably the mines of the Sus and Daï valleys in present day Morocco). What likely happened was that these long brass bars were used in the Tegdaoust workshops to dilute the locally produced and more expensive copper, without altering the color of the metal. From coming in long shapes, formed to fit being carried by camels over a long distance, they were melted, blended and formed in smaller shapes to fit donkeys that carried them south to the gold-rich areas of modern day Guinea.
Laurence Garenne-Marot further speculates that, following disastrous raids on Tegdaoust, the north-south supply lines as well as copper money production shifted east towards Kumbi Saleh; something that might explain the relatively large number of small copper bars in that town stemming from a period after the raids.
The way some of the value protocols worked in the area is now becoming clearer. Although gold was relatively prevalent all over Muslim West Africa, as well as in the North African nations, it was still of course highly valuable. Mansa Musa's account of incredible copper-to-gold exchange prices in certain areas, combined with the copper- and brass production in the Empires of Ghana, Mali and in North Africa, made for an obvious result; Muslim merchants bought copper locally as well as imported copper and brass in order to export it to gold producing tribes. As copper ore on a large enough geographical area is far easier to find than gold ore, this ultimately caused gold to flow out of any place where it was locally undervalued relative to copper. Consequently, by relatively overvaluing copper, the gold-rich tribes were flooded with the (often diluted) corrosion prone metal. This was not really Gresham's Law in action as we don't know what legal tender laws, if any, were in place, but it does paint the picture of the market mechanism working it's wonders on commodity 'mis-pricing'. The 'bad' copper money drove out 'good' gold money due to local gold abundances in relation to global gold scarcities. Laurence Garenne-Marot argues in his essay that the geographical extreme differences of the copper-to-gold exchange ratio may have also had a cultural basis. This may be a prime example then of where money as a social or cultural phenomenon can be more or less wiped out anyway in the face of economic reality (physical scarcity). Any tribe that socially converged on storing value on the copper based value protocol would see their wealth evaporate as the northern tribes could produces those monetary units with relative ease.
Curiously enough, in the earlier mentioned abandoned brass bar caravan, large amounts of cowrie shells were found as well. It does not demand much imagination to consider that an intelligent merchant, if diluting the southern copper money supply too much, also would have liked to sell other, more scarce types of money for more profits. As most of the Ghanaian and Malian financial centers lay inland, far from any coast, cowrie shells might indeed have been very scarce during certain periods. Battuta mentioned such shells in his writings:
These [cowrie] shells are used also by the negroes in their lands; I saw them being sold at Málli and Gawgaw at the rate of 1 150 for a gold dinar. 
Such value protocol has the same relative easiness as did the copper; it is easy to imagine a scenario where cowrie shells trade for a high gold price at the bazaars of some desert towns, but as you conceptually widen the geographical area, it becomes more and more likely that it includes coastal towns or villages that can produce these cowrie shells at a higher rate that what the same area, due to it's physical scarcity over and under ground, can produce gold. Any tribe storing wealth on a cowrie shell protocol over a long enough period of time would, facing economic reality, see it wiped out by harder money through simple trade mechanisms.
With the above examples, I hope to have shown that the convergence of people towards certain value protocols have little to do with any other factor than scarcity. It didn't matter how pretty the cowrie shells were. It didn't matter how useful the copper were. Money in a truly free market is not the result of social construction, but rather of social exploration. This exploration ultimately converges on a protocol with certain fundamental properties, where the most fundamental of all is hard scarcity.
 Ibn Battúta (1325-1354). Travels in Asia and Africa. Translated by H. A. R. Gibb. London: Routledge & Kegan Paul Ltd., p.336.
 Garenne-Marot, L. (2009). 'Fils à double tête' and Copper-based Ingots: Copper Money-objects at the Time of the Sahelian Empires of Ancient Ghana and Mali. British Museum Research Publication 171. Garenne-Marot refers to: Al-'Umari, in Levtzion and Hopkins 2000, 272, ref. manuscript B 67, 36a.
 Ibn Battúta (1325-1354). Travels in Asia and Africa. Translated by H. A. R. Gibb. London: Routledge & Kegan Paul Ltd., p.243.