The stone extracted from these quarries is a white marble of a close fine grain. It is readily distinguished from Parian marble—the other white marble commonly used by Greek sculptors and architects—by its finer grain and opaque milky whiteness; whereas the Parian marble is composed of large transparent crystals, and is of a glistering snowy whiteness. Parian marble resembles crystallised sugar; Pentelic marble resembles solidified milk, though its surface is of course more granular. Pentelic marble, alone among all Greek marbles, contains a slight tincture of iron; hence its surface, when long exposed to the weather, acquires that rich golden-brown patina which is so much admired on the columns of the Parthenon and other buildings constructed of Pentelic marble. The Parian marble, on the other hand, though it weathers more easily than the Pentelic on account of its coarser grain, always remains dazzlingly white. Pentelic marble is always clearly stratified, and in places it is streaked with veins of silvery white, green, and reddish-violet mica. Blocks so streaked were either thrown aside by the ancients or used by them for buildings, not sculpture. But even in architecture these veins of mica entailed this disadvantage that the surfaces containing them, when long exposed to the weather, split and pealed off in flakes, as we may see on the drums of the columns of the Olympieum or Parthenon. Besides the fine white marble already described, which is commonly known simply as Pentelic marble, there occurs on Mount Pentelicus a grey, bluish-grey, and grey-streaked marble identical in kind with the marble known as Hymettian, because the ancients quarried it on Mount Hymettus. This grey or bluish-grey marble is of more recent geological formation than the white. It does not appear to have been quarried by the ancients on Pentelicus; at least no ancient quarries of it have been discovered on the mountain. But it is now obtained in great masses in the large modern quarries to the east of Kephisia, and furnishes Athens with building material for the better class of houses and public edifices; even paving-stones are made of it.