The Unfinished Obelisk.

One of the most famous stones left behind is the Unfinished Obelisk, taller than any known obelisk ever raised. Quarrymen apparently abandoned the obelisk when fractures appeared in its sides. However, the stone, still attached to bedrock, gives important clues to how the ancients quarried granite. Much of the red granite used for ancient temples and colossi came from quarries in the Aswan area (500 miles south of Cairo). The Unfinished Obelisk still lies where a crack was discovered as it was being hewn from the rock. Possibly intended as a companion to the Lateran Obelisk, originally at Karnak, now in Rome, it would have measured 120-feet and weighed over 1150 tons when complete.

The obelisk's creators began to carve it directly out of bedrock, but cracks appeared in the granite and the project was abandoned. Originally it was thought that the stone had an undetected flaw but it is also possible that the quarrying process allowed the cracking to develop by releasing the stress. The bottom side of the obelisk is still attached to the bedrock. The unfinished obelisk offers unusual insights into ancient Egyptian stone-working techniques, with marks from workers' tools still clearly visible as well as ochre-coloured lines marking where they were working.(1)

How was it done: It is now known that the main tool employed for carving the granite were small balls of Dolerite which is a mineral harder than granite, as seen at the open air museum/quarry at Aswan, Egypt today. The discovery of this obelisk and several others in their unfinished states allows us to see how they were made. The means of separating the stone from the bedrock was a common technique used around the ancient world, in which small cavities were made in the stone, which were then filled with wood, which was soaked in water causing it to expand (See photo, right. Aswan, Egypt)

(Other examples of this from around the ancient world)