Senator SMITH. Do you know what an iceberg is composed of?
Mr. LOWE. Ice, I suppose, sir.
Senator SMITH. Have you ever heard of an iceberg being composed not only of ice but of rock and earth and other substances?
Mr. LOWE. No, sir.
Icebergs can contain rocks and soil, like the glaciers they come from. Glacier ice itself is a rock, according to this USGS article -- it's a mono-mineralic rock (as opposed to rocks that are mixtures of minerals) with the formula H2O. I'm neither a geologist nor an engineer, but thinking of the iceberg as a floating rock might be a helpful simplification. The iceberg was big enough, heavy enough, and hard enough to damage Titanic the way it did. Here's a simplified model: the iceberg forced a change in the ship's velocity, and the energy 'released' by this change had to go somewhere. That energy couldn't move the iceberg enough (it was an inelastic collision), it couldn't shatter the iceberg enough, so it deformed Titanic's hull. There isn't any need for abrasion.
Perhaps that helps -- mostly, I just wanted to say "Ice, I suppose."
Strength and hardness of ice is also a function of its age, but its homogeneity is also important because it allows the crystal structure to develop into the greatest strength. Ice basically cold-hardens when exposed to extreme temperatures in the cryosphere.
Icebergs form in the glaciers of Greenland and advance during the winter forming valleys and fracturing the rock as their advance, engulfing them in the ice. During summer, the outter layer of the glaciers melts and breaks away forming an iceberg. It is most probable that the iceberg that sunk Titanic also contained rocks.