Most people regard diamonds simply as expensive pieces of jewellery. But for New Yorkers Alan Bronstein and Harry Rodman, whose Aurora Pyramid of Hope of 296 naturally coloured diamonds is currently on display at the Natural History Museum, they represent something far more significant.

"It might sound crazy," says Bronstein, "but I see the collection as a picture of all the races, all the colours, all the different religions and faces of humanity.

"There are different cuts and shapes, but they are all representative of the one family - the diamond family."

Bronstein has painstakingly added to the collection over a 25-year period, and the diamonds on display total 267 carats.

His affinity (he would say "spiritual communication") with diamonds developed when, as a young man in the 1980s, he took what he thought would be a temporary job after graduating from college. His mother was the book-keeper of the Diamond Dealers

Club of New York and she suggested that Bronstein become a diamond broker as a way of earning a living until he decided what he really wanted to do.

He remained a broker for three years until the life-changing moment when he saw his first coloured diamond. "It was a turning point for me," he says. "From that time I decided I would like to begin a collection. I knew that this was a path I would like to follow come what may."

He started to acquire coloured diamonds partly for business, but mainly because they had become his passion.

At first, they were relatively inexpensive (20 years ago coloured diamonds were little known or valued compared to their clear counterparts), but gradually he began to obtain more prized stones. In 1986 he joined forces with his patron Rodman, a retired gold refiner and D-Day veteran who still takes a close interest in the collection even at the age of 96.

Bronstein had very clear goals for the stones.

"I knew we couldn't just go out and buy every million-dollar diamond ? it was a long project. We were trying to buy diamonds that would give us another piece of the puzzle we were trying to put together."

Part of a Jewish tradition of diamond cutting and trading which goes back to 17th-century Amsterdam, Bronstein admits that there are still diamonds he would love to add to the collection. He has yet to lay his hands on a stone the colour of a red traffic light, for example.

"To even see one would be a blessing. My assumption is that if God can make all the colours, then He can make that colour," he says.

One aspect of the collection he refuses to discuss is its value. "We can't say how much it's worth, just that it's somewhere between $1 million and $100 million. It's not about money. I want people to see it and be stunned in the same way as the first time they saw a rainbow."

"Diamonds" is at the Natural History Museum until February 2006. Tickets are available on 0870 013 0731

Photo credit: Natural History Museum of London