Occurrence
Corundum does not occur in association with the zinc ores but is widely distributed in isolated pockets in the Franklin limestone. The principal localities for the mineral in Sussex County are not in the Franklin district. In the pockets the corundum is associated with one or more of the minerals spinel, rutile, graphite, edenite, pyroxene, garnet, titanite, and phlogopite.

In excavating in limestone for the foundation of the furnace on the west side of the pond at Franklin, a pocket was opened that contained many crystals of blue, gray, and red corundum, some of them transparent and very sharply crystallized. With the corundum were phlogopite, spinel, rutile, and marcasite, but the marcasite crumbled soon after exposure. Similar blue and gray corundum crystals, like those shown in Plate 4, B, were obtained from a pocket near the limestone quarry of the Franklin Iron Company.

Figure 29 End of prismatic crystal of corundum showing the forms c(0001), a(1120), n(2243), k(7.7.14.6), r(1011), h(0111). Calamine pit, Sterling Hill.
	Figure 30 End of prismatic crystal of corundum showing the forms c(0001), a(1120), n(2243), k(7.7.14.6), r(1011), d1(0112). Furnace quarry, Franklin.

In pockets near the western border of the ore body of Sterling Hill and in the loose material from the stripping of the calamine pits were found good crystals of blue and red corundum associated with rutile. Many of these were found in washing the calamine ore for the removal of dirt and were doubtless derived from pockets in the neighboring limestone. An irregular chain of pockets containing corundum is said to have extended from Sterling Hill to Franklin.