It’s big. It’s green. It’s oval. It weighs 25 pounds. Is it a watermelon? No, it’s an emerald! In fact, some say, it’s the world’s biggest emerald—and it’s worth at least 190,000 ripe melons, give or take a few.

The giant “gem” has been nicknamed “Teodora,” a Portuguese word that means “gift of God.” It weighs 25 pounds—about as much as a medium-size dog. Teodora is in the news because it is likely to be sold at an auction in Canada on Saturday. There’s a good chance it will fetch more than its estimated $1.15 million value.

But surprisingly little is known about Teodora’s history. That’s one reason that some gem experts doubt that it’s a real emerald at all.

A BIG GREEN FAKE?

Emeralds are a green form of the mineral beryl, which is usually white. White beryl is so common that it’s worthless. But green beryl–emeralds–is rare and worth a lot of money. The darker the green, the more an emerald is likely to be worth.

As a result, many dishonest people dye white beryl green. Then they pass off their fakes as emeralds. Some gem experts say that Teodora is too big to be a real emerald. They think it must be white beryl that has been dyed.

Jeff Nechka, a gem expert in western Canada, was brought in to test Teodora to see if it’s real. “At first I was very skeptical,” says Nechka. “Anything this large occurring in nature is extremely rare.”

Nechka did find evidence that Teodora had been dyed. However, he says other factors—like its structure—show that it started off as a genuine green emerald. Someone simply dyed it a darker shade of green in an attempt to make it worth more. Nechka is the one who estimated that Teodora is worth $1.15 million, even with the dye job.

Both Nechka and Teodora’s current owner, Regan Reaney, say that it could be sold at auction for much more.

Why buy a 25-pound emerald? "Obviously, you can't make a piece of jewelry out of something this big," Reaney explains. "Whoever buys it will probably put it on show in their home or a museum or try to resell it for more.”