Atomic impurities in diamonds, specifically negatively charged nitrogen vacancy centres in those diamonds, could be used for extremely long term storage.  Researchers have used optical microscopy to read, write and reset the charge state and spin properties of those defects.  This would mean that you could store data, in three dimensions, within these diamonds almost perpetually.  There is one drawback, as the storage medium uses light, similar to a Blue-Ray or other optical media, exposure to light can degrade the storage over time.  You can read more about this over at Nanotechweb.

"The nitrogen vacancy (NV) centre can be used for long-term information storage. So say researchers at City University of New York–City College of New York who have used optical microscopy to read, write and reset information in a diamond crystal defect."

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