What is this picture?

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What is shown in this image?

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mussel shell nacre. # Mussel Shell Nacre

Nacre (that shimmering inner layer of mussel shells) is about 3,000 times tougher than the raw materials it's made from—it achieves this superhero-level durability by arranging microscopic aragonite crystals like a brick wall, with an organic glue between each "brick" that absorbs impact. Materials scientists have been studying nacre for decades to create next-generation body armor and smartphone screens that won't shatter, essentially trying to steal nature's 500-million-year-old engineering secret.

Answer: mussel shell nacre