Evanescent [Houseki no Kuni]

A long, long time ago, sentient and highly intelligent creatures lived on this once greener planet. They built, they conquered and they advanced. They were called "humans". Their technology had reached a point that it could predict large and small astrological events with pinpoint accuracy, however, the machines... withheld the truth. And, quite soon, six meteorites hit the planet Earth, creating six distinct moons and erasing 99% of the planet's landmass. Only one island was left in the destruction's wake.

Now, 41,455 years later, strange, lustrous beings inhabit the island. Their bodies have simple yet fascinating anatomy. They... are made up of gemstones. Immortal. Unchanging and irreplaceable. But, they are not safe. Mysterious creatures called "Lunarians" often attack them. They come from the moon, a place where should one of these Lustrous find themselves in... there is no salvation.

Such is the fate of those who exist forevermore...
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[In this book, the other Gems excluding the MC are female.]