Richard LaMotte knew he had a winner when he spotted a small shard of aquamarine glass one day sitting amid the reeds, rocks, and oyster and clam shells that line the shore near his home on Maryland's Chesapeake Bay.

It turned out to be the heel of a Coca-Cola bottle, smoothed and tumbled for decades by fresh and saltwater waves into an elegantly frosted nugget. LaMotte estimates that it dates back to the 1930s, when the bayside town of Tolchester, Md., attracted tens of thousands of sun seekers from Baltimore and elsewhere every weekend.

“It's the size of a thumb and perfectly worn," said LaMotte, the author of Pure Sea Glass, a color-photograph reference guide to finding and collecting sea glass. His wife Nancy, a jewelry designer, uses the sea glass in her work, he said, but he decided to keep this piece for himself.

“I told her I didn't want her to make jewelry out of that one," he said with a chuckle. The piece is now displayed in a custom cabinet along with other treasured specimens the couple have found over the last decade -- a perfectly worn red-hued nugget from the edge of a warning light, a smooth piece of lavender sea glass from an antique perfume stopper, a yellow glass knob in the shape of a bear's head that may have belonged to a century-old jar lid.

Since its patenting in late 1915, the Coca-Cola contour bottle has been widely celebrated, from its appearance on the cover of Time magazine in 1950 to its immortalization by artists like Andy Warhol. Now, as the bottle marks its 100th anniversary, it has picked up a new set of admirers: the growing number of sea glass collectors who cherish its aquamarine green color and the nostalgia that comes with a century-old possession.

"It's frosty, sea-foamy and beautiful like the ocean. It's also nostalgic for a lot of people," said Mary Beth Beuke, founder and former president of the North American Sea Glass Association and the author of The Ultimate Guide to Sea Glass. 

An estimated 75 percent of sea glass comes from bottles, and Coke bottles make up the majority of the burnished antique green sea glass found on beaches from Maine to Japan. That's due in large part to the company's use of the color green in its glass bottles, LaMotte said.

"Automation came in the 1900s and bottling companies switched" to clear and brown colors for efficiency reasons, he said. "It wiped out all the blues and greens. Coke was the exception. It continued with that nice soft green color from 1915 until well into the 1960s."

Prior to 1915, Coca-Cola bottles were amber or a clear hue known as flint, explains Ted Ryan, Coke's director of heritage communications. When the company standardized and went with the contour bottle, also known as the "hobble-skirt" or "Mae West" bottle for its hourglass curves, it wanted to standardize on a color as well and chose a light green color also known as German green, he explained. It was later called Georgia green in homage to the company's home state.

“Nobody else had ever done this up to that point. So they picked a green, the green we all know today," Ryan said. “We religiously enforced the use of the green dye. If we caught bottlers going back to flint, which was cheaper, we would make them come back to that proper green. It became the standard color of the company."

If you're lucky, collectors say, you might find a piece of a Coke bottle that still has the raised lettering of the patent date and year and name of the town where it was bottled, or the raised horizontal lines that run from the neck to the heel of the contour bottle. These distinctive marks are rare for sea glass, Beuke notes, since decades of ocean tumbling tend to smooth it all away.

Sea glass essentially comes from jars, bottles, tableware and other items that were discarded into the ocean via coastal landfills, shipwrecks, and the pre-recycling habits of people in the late 1800s and early 1900s. Decades of wave action, or natural tumbling, turn it into the pretty, smooth and frosted nuggets sought out by jewelry designers and collectors.

Recycling, returnable bottles and the increased use of plastic have made sea glass like LaMotte's treasured Coke-bottle heel harder to find, but that may have helped fuel its popularity in recent years, collectors say.

"People see that it's going away, so a lot more jewelers are buying and paying more for sea glass jewelry because they can't find it anymore," LaMotte said. The Internet and glossy magazine and newspaper features have also helped expand its fan base, he said.

In Japan, contour Coke bottles have gone through a similar artistic rebirth of sorts. Coke shipped thousands of bottles to servicemen stationed overseas during World War II, including Okinawa in the Ryukyu Islands. When residents returned their war-torn island, some resourceful artists began collecting discarded Coke bottles and repurposing them into everything from simple glass bulbs to exquisitely patterned vases. Known as "Ryukyu glass," it has developed into a trademark craft on the island and a major tourism attraction.

More recently, designer Oki Sato transformed the contour bottle into elegant green glass tableware as part of Coca-Cola Japan's ongoing program in design sustainability.

Once favored largely by jewelry-makers, sea glass is now being used in all sorts of creative ways, from swimming pool decor to wall hangings and wind chimes, as more people embrace its mix of beauty and history, Beuke notes.

"It represents something historic that's been on a long journey," she explained. "That's where the life metaphor comes in: this is something that was made more beautiful after having gone through a harsh environment, but it comes out pretty and softened at the end."