CPU underside discoloration.

kol12

Honorable
Jan 26, 2015
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I've noticed on the underside of my 8700K CPU that some of the gold contacts are slightly discolored or darker looking than the others. Can anybody tell me what causes this? Is it normal or anything to be concerned about? Is this something that happens to the CPU over time or can the contacts naturally have a slight variance in colour from the get go? My dark spots look similar as to what is seen in this 7700K:
Intel-i7-7700-K-04.jpg