The scene: Earth, but with a single major modification, the addition of a HUGE quantity of a single element to the Earth's crust (bringing it up to the same abundance as Iron in the crust).
The element: The added element must have a density equal to, or greater than, Palladium. It must be stable under standard conditions. It must occur naturally in the real Earth's crust already, though it need not be in a native state, ores are ok. It must not be hazardous to humans (eg. no Mercury or Plutonium). It must be something that would have a significant influence on pre-1500 (the year 1500 or earlier) metalworking technological advancement, though it need not be a metal itself (eg. Carbon is not a metal but has a huge significance in metals tech).
The actual question: Which element (meeting the above criteria), if it had been so much more abundant and available in the crust than it actually is, would have caused the most dramatically different development in metalworking than what actually happened in history?
EDIT Apparently the purpose of the question is regarded by potential answerers as more important than I originally anticipated. So to explain it briefly: I'm building a world that is smaller but more dense than Earth, to yield earth-like gravity. I'm doing this by adding more dense material(s) to it. This question was intended to help me understand some of the likely implications of this element being available in greater quantities than on earth to technological advancement of civilizations that are placed on this world for the story, which takes place in a roughly medieval tech level. Emphasis added to "in the crust" to indicate that changes to the overall density to Earth, and gravity, etc., are outside the scope of the question. Other than the element is more available for metalworking/metallurgy/etc., other effects on the Earth can be disregarded.