They’ve already hurtled millions of miles through outer space. But later this month, some of them will take a shorter, more earthly journey—home with the highest bidder. On April 20th, Christie’s London is putting 83 meteorites up for sale, each with their own unique look, composition, and origin story, the Daily Mail reports.

It took a lot of creative number-crunching to get these rocks on the auction block. While meteorite salespeople used to price their wares strictly by weight, recent years have seen a strategic overhaul. When determining value, “we use the four S’s: size, shape, science, and story,” auction organizer James Hyslop told Bloomberg Business. Meteors that shot straight through the Earth’s atmosphere, and thus melted into a “perfect parabola,” will go for more than those that somersaulted in willy-nilly, and look messier.

Pieces of the moon or Mars also sell well, as do those rocks that are studded with peridot. One gemstone-filled parabolic meteorite is expected to sell for $1.1 million. (In contrast, the Valera meteorite, famous for being the only space rock to have killed a cow, will likely fetch less than $10,000, reports Artnet News.)

The 83-rock haul is up for viewing April 16th through 19th at Christie’s in South Kensington, before going on sale on the 20th at 2:30 PM.

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