From the Middle East the Chinese acquired a blue pigment—a purified form of cobalt oxide unobtainable at that time in China—that contained only a low level of manganese.

中国人从中东获得了一种蓝色颜料，这是一种当时在中国很难得到的纯净的氧化钴，这种颜料只含有少量的锰。

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Instead, people with blue eyes have a brown pigment, known as melamin, at the back of their irises but have low concentrations of melanin in the front of their irises.

相反，蓝眼睛的人有一种褐色色素，被称之为黑色素，位于虹膜背部，但在虹膜前部黑色素浓度较低。

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A blue eye pigment doesn't actually exist in nature.

蓝眼色素在自然界实际上并不存在。

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The blue pigment Mayans used to paint temple walls 1, 400 years ago was made of particles only about 50 nanometers across.

FORBES: The Science of Small

The new work was begun during a conservation treatment in 2009, when conservators found that the yellows in Flowers In A Blue Vase - this time from a pigment called cadmium yellow - had turned greyish and cracked.

BBC: Van Gogh's Flowers In A Blue Vase damage seen in X-rays

The mutation that causes blue eyes reduces the expression of OCA2 and hence reduces pigment concentration.

WSJ: Matt Ridley on Human Culture's Effect on Genes | Mind & Matter