'Whale vomit' could fetch $70,000

If you happen to come across a smelly rock while taking a long walk on the beach, it could be your lucky day.

A couple in the UK stumbled across a large rock that many are calling "whale vomit." The "rock" could be ambergris, which is a rare substance used to make perfumes last longer on the skin.

"Ambergris is definitely not vomit," Christopher Kemp, author of "Floating Gold: A Natural (and Unnatural) History of Ambergris". "It's more like poop, and it comes from the same place as poop, but it's only made by a small percentage of sperm whales, as a result of indigestion."

Gary and Angela Williams followed a pungent smell while walking on Middleton Sands beach near Morecambe Bay that led them to what they believe to be a large lump of ambergris.

"Ambergris feels a little waxy, and smells very complex: a mixture of dung and the ocean, and old wood, and tobacco, and moist earth, and ozone," Kemp said.

The substance, often called "floating gold," is produced only by a tiny percentage of sperm whales. It can float in the ocean for decades until it eventually washes up on shore, where people like the Williamses find it.

Before going out to scour the nearest beach to find ambergris, be warned that it is extremely rare and found infrequently. And it's very hard to know if you've actually found ambergris, Kemp said.

Kemp suspects that the couple's find may not be genuine ambergris. While ambergris is waxy, the substance they found is "a little too waxy" and looks more like animal fat than ambergris, he said.

If it is genuine ambergris, the British couple's lump could be worth an estimated $70,000. They are reportedly in negotiation with potential buyers in New Zealand and France.

In 2012, an 8-year-old British schoolboy found a 1.3-pound mass of it in the sand that was worth about $63,000. Good, high-quality ambergris is worth thousands of dollars per pound, Kemp told CNN.

Although the harvesting of ambergris isn't harmful to the whales, the trade of it is banned in the United States in an effort to not exploit the endangered species.