This is the largest known fragment of the True Cross in the world.
It is treasured at the Holy Monastery of Xeropotamou, Mt. Athos. A hole from the nail that went through our Saviour’s right hand, can be seen surrounded by diamonds at the lower portion of the vertical cross-bar.
source: http://www.orthodoxiconsonline.com/

A little-known fact about the holy mountain is that, besides housing many another Christian relic, its monasteries contain more pieces of the True Cross than any other single locality. The Cross has been calculated to have measured 178,000,000 cubic millimetres. Of 3,942,000 c. mm. extant, Mount Athos has pieces aggregating 878,360 c. mm., Rome 537,587 c. mm.
Source: article in Time magazine january 21 1935

Gerasimos Smyrnakis[1903] notes that the largest surviving portion, of 870,760 cubic milimeters, is preserved in the Monastery of Koutloumousiou on Mount Athos, 537,587 cubic milimetres in Rome, 516,090 in Brussels, 445,582 in Venice, 436,450 in Ghent and 237,731 in Paris.
Source: wikipedia.
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