The solid silver cross was found by Mr Kane while he was using a metal detector in the field at a farm near Grendon.
The father-of-two had been scouring the land at Grendon House Farm, which is owned by Jim Brodie in the village's Main Road, for more than two hours when he found the crucifix, which dates back to the 1200s, two inches under the surface.
Experts from one of London's main auction houses will now be called in to establish the value of the medieval crucifix, which is just under an inch-and-a-half long and has the letters IESVS inscribed on it which is the medieval word for Jesus.
Mr Kane, age 42, of Shire Place, Overstone Lodge, Northampton, said: "I wasn't exactly sure what it was when I found it, but when I cleaned it up I could tell it was silver."
Mr Kane has been metal detecting for 10 years and is a member of two Northamptonshire clubs and when he is not working as a carer, he spends about 12 hours each week looking for finds.
After he discovered the silver cross he reported it to his metal detecting club, which informed the British Museum.
Mr Kane said: "I've found quite a lot of other stuff over the years, but the cross is the only thing I've found that's been really valuable.
"I've found a few 300-year-old coins in the same field though.
"I think there was a market there in medieval times and I've also been told there might once have been a battle in the field, so I've had quite a few good finds there."
Beverley Nenk, the curator of the medieval collection at the British Museum, said: "It would have been used as an item of personal jewellery, and would have been bought by someone fairly affluent. It would have been used to ward off evil spirits and harm, and would have had the same symbolic significance as a modern-day crucifix."
According to Ms Nenk, the last time a similar example was found was five years ago at Beachamwell in Norfolk, which was valued at £500.
Once valued, Mr Kane and Mr Brodie will be in line to split any money paid for the crucifix, and Kettering Museum has expressed an interest in buying it.
The cross was found on November 4 last year, and an inquest yesterday deemed that it should be classed as treasure under the Treasure Act of 1996.