A brown leather-bound book kept under the number P.D. 66c in Museo Civico e Raccolta Correr di Venezia, is an unpublished accounting ledger, which belongs to the commenda agent of Minasian family firm, Agha di Matus (1644 - 1709). This unique ledger contains information about the cities in which he traded between 1679 and late 1680s and has not been published or closely scrutinized before. The consignments, described in the accounting ledger, contained diamonds and rubies, pearls, turquoise, lazurite, etc, all shipped or brought to Agha di Matus personally in Venice by different agents of the Minasian family firm trading in the Indian subcontinent, Izmir, Aleppo, Constantinople, Baghdad, Venice, Livorno, Amsterdam, Marseille, London, etc. Diamonds and gems formed some of the most lucrative global commodities of the early modern period and provided an important link between the production or mining centers in the Mughal Empire and the consumption centers in Europe. This trade was largely conducted by Armenian merchants from New Julfa as well as their counterparts from the Sephardic trade diaspora in Europe, and the various chartered East India companies of Europe. Like other Asian mercantile communities of the period, the Julfans organized their business ventures around the economic institution known as the “family firm.” Julfans did so by combining the archaic structure of the patriarchal family with modern techniques of investment and credit transactions. My paper will examine Agha di Matus’s life as a diamond merchant and the history of the Minasian family firm, thus shedding light on the important yet largely neglected history of this Armenian family firm and their most significant agent. It will treat Agha di Matus as broadly representative of a group of other Armenian merchants involved in the diamond and gem trade of the early modern period. Based on the above-mentioned accounting ledger and around a four dozen mercantile letters from four different archives, it will explore the various ways in which Agha di Matus and other agents of the Minasian family network traded with gemstones on the caravan trade routes of Safavid Iran, followed by the maritime routes stretching from Izmir to Venice and Livorno. By exploring the details of participation of Julfan diamond merchants in the commodity trade between India and the Mediterranean, this paper seeks to fill in the gap in the existing scholarship of the early modern diamond trade and the involvement of Armenian merchants in it.