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Detail of the aft (active) side of the Gemini heatshield subjected to reentry-type temperatures. See the NASM collection ID #A19781814000 page, which notes that this heatshield "may have been installed on Gemini 2").

In this view, it is more obvious that the inserts placed in the core sample holes are merely similarly-colored (which are even more obvious from the opposite side).

To see what a Gemini heatshield looks like after a core has been removed, see this photo on Gemini 12 at Adler Planetarium.