Coltan is the colloquial African name for columbite-tantalite, a metallic ore used to produce the elements niobium and tantalum. The endoskeletons of advanced Terminator machines are made using alloys derived from a half percentage of coltan to make them hardened to heat. 
Cameron sends the shipment dramatically off the edge of a steep cliff into deep waters, preventing those 530 endoskeletons from being manufactured post-Judgment day. She does, however, retain one Coltan rod, perhaps for future repairs of her own endoskeleton.
Zeira Corporation owns 7% of the Earth's Coltan reserves.
- John: That's coltan; if that's what you're made of, no wonder you're so dense.
- Cameron: Not density, heat resistance. T-600 models had a titanium alloy endoskeleton, but it was vulnerable to heat. Coltan alloys have a much higher melting point.
- The Sarah Connor Chronicles, "Heavy Metal"
- Coltan has recently incited a war in Africa 
- Coltan is known as the blood-metal of Africa.
- Coltan, as the key component in endoskeleton alloys, was a concept first introduced in TSCC, though the production team for Terminator Salvation mistook this for canon established in the original movies and therefore included coltan as the alloy for the T-800s in their movie.