Samples of the previously molten metal
Photo below shows a large piece of the now-solidified metal with entrained material, stored (as of November 2005) in a warehouse in New York. From Dr. Steven Jone's paper Why Indeed Did the WTC Buildings Completely Collapse?. The image is divided horizontally in two pieces, click on either one to examine it more closely.
Rust indicates the presence of iron and/or steel. This sample of previously molten metal has inclusions of what appear to be reinforced concrete and likely other materials. Some have suggested that some or all of this metal could be aluminum but aluminum does not alloy with steel nor does it rust like steel. There are however some granular areas top center of the top photo above which look like they might be small deposits of aluminum which would be expected to float on top of any molten steel. A great deal of forensic analysis remains to be performed on this and the other chunks of molten iron/steel found in the basements of WTC 1, 2, and 7.
ANOTHER chunk of molten iron/steel has been preserved for (hopefully) later forensic analysis, this one has less rust but is very dense with fewer concrete inclusions and is called The Meteorite.
There are probably better pictures of The Meteorite, the one above was pulled out of a good YouTube video which summarizes a lot of discussion and evidence about the molten iron/steel found:
9/11: WTC High Temperatures & Molten Steel