Coltan is the colloquial African name for columbite-tantalite, a metallic ore used to produce the elements niobium and tantalum. The endoskeleton 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.