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Is it true that, before the Islamic period, the Dirham constituted any silver currency, be it coinage or not?
http://www.iranica.com/articles/dirham says, "DIRHAM (< Gk. drakhmē´ “drachma”; Mid. Pers. drahm, Pers. derham), a unit of silver coinage AND of weight" and, in "ii. In the Islamic Period" section, "For Muslims in the classical period, ANY silver coin was a dirham, and a dirham was ALSO A MONETARY UNIT THAT MIGHT OR MIGHT NOT BE REPRESENTED BY A CIRCULATING COIN. A dirham was ALSO A SMALL WEIGHT UNIT, USUALLY NOT THE SAME AS THE WEIGHT OF A MONETARY DIRHAM".
If you can answer, would be very appreciated 
So basically what I mean is, was it not a specific coin or currency before the second caliph (Umar bin al-Khattab) made it official?
I found this on Wikipedia for you:
"Historically, the word "dirham" is derived from the name of a Greek coin, the Drachma (δραχμή); the Greek-speaking Byzantine Empire controlled the Levant and traded with Arabia, circulating the coin there in pre-Islamic times and afterward. It was this currency which was initially adopted as an Arab word; then near the end of the 7th century the coin became an Islamic currency bearing the name of the sovereign and a religious verse. "
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