Pirenne, Henri: The Expansion of Islam in the Mediterranean Basin 
We see from the letters of Saint Boniface how rare and expensive spices had become. He received or sent presents which consisted of small quantities of incense.(57) In 742-743 a cardinal sent him aliquantum cotzumbri quod incensus, Domino offeratis.(58) In 748 an archdeacon of Rome also sent him a small consignment of spices and perfume.(59) These gifts prove the rarity of spices to the north of the Alps, since they constituted valuable presents. We should note, however, that they all came from Italy. They were no longer reaching the port of Marseilles. The cellarium fisci was empty, or even—which is very probable—had been burned by the Saracens. And spices were no longer an article of normal commerce. If small quantities still found their way into the country, it was by means of pedlars.
In all the literature of the period, although this is very abundant, there is hardly any mention of spices.
It may be asserted, in view of this scarcity, that by the end of the 7th century and the beginning of the 8th spices had disappeared from the normal diet. They did not reappear until after the 12th century, when the Mediterranean was reopened to commerce.
The same thing applies, of course, to the wine of Gaza, which also disappeared. Oil was no longer exported from Africa. Such oil as was still used came from Provence. Henceforth the churches were lit with wax candles.
Similarly, the use of silk seems to have been almost entirely unknown at tins period. I find only one mention of it in the capitularies.(59a)
We know how simply Charlemagne was accustomed to dress. The court would certainly have imitated him. But no doubt this simplicity, which contrasts so strongly with the Merovingian luxury, was a matter of necessity.
We must conclude, from all this that imports from the Orient had ceased in consequence of the Islamic expansion.
We note another very striking fact: the increasing rarity of gold. We see it in the Merovingian gold coins of the 8th century, which were alloyed with a constantly increasing proportion of silver. Evidently gold had ceased to arrive from the Orient. While it continued to circulate in Italy, it became so rare in Gaul that it was no longer employed as currency. From the time of Pippin and Charlemagne only silver denarii were struck, with very rare exceptions. Gold resumed its place in the monetary system only when spices resumed theirs in the normal diet.
This is an essential fact more eloquent than all the texts. It must be admitted that the circulation of gold was a consequence of commerce, since where commerce survived—that is, in southern Italy, gold continued to be in common use.
One consequence of the suppression of the Oriental trade and maritime traffic was the disappearance of professional merchants in the interior of the country. Henceforth merchants are hardly ever mentioned in the documents of the period; any references that do occur may be understood as applying to occasional merchants. I can find no mention at this period of a single negotiator of the Merovingian type: that is, a merchant who lent money at interest, was buried in a sarcophagus, and gave of his goods to the churches and the poor. There is no evidence whatever of the continued existence in the cities of colonies of merchants, or of a domus negotiantum. There can be no doubt that as a class the merchants had disappeared. Commerce itself had not disappeared, for we cannot imagine a period without any sort of exchange, but it had assumed a different character. As we shall presently see, the spirit of the age was hostile to it, except in the Byzantine countries. Moreover, the fact that very few laymen were able to read and write rendered impossible the continued existence of a class living normally by sale and purchase. The disappearance of loans at interest affords further evidence of the economic regression produced by the closing of the sea.