Drawing comes naturally and willfully to Alex Koch, whose canvas for his drawings of birds, fish, winter scenes, lighthouses and other objects is not cloth or paper but rounded white rocks collected on beach hunts.

What began as a hobby for the Mattituck teenager has become an online business: North Fork Rock Art, a response to increasing demand for his creations. Alex, 13, has been drawing for about a decade.

“I started when I was 3 or 4, as soon as I was physically able, because I loved it so much,” he said.

Alex never drew on walls or furniture, only on paper. He generated stacks of paper with his artistic flair, but something was still missing. Then one day his mother, Michelle Koch, pointed to some rocks already polished and shaped by nature during a summer walk on a beach in Hampton Bays.

“My mom said, ‘Why don’t you try to draw something on one of these rocks?’ ” Alex recalled. “I made a sandpiper that came out so well I started making more.”

“Everybody loved them,” his mother said.

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Last July, she and Alex set up North Fork Rock Art online, displaying his work and taking orders on Facebook. They have sold nearly 500 rocks so far.

Scenes, such as a pagoda-shaped lighthouse set against an orange sky dotted with stars; herons and other birds in flight and at rest; a cottage with snow on the roof and a snowman in the front yard, elicit raves on his Facebook page: “Awesome Alex! I love it.” “Your creations are amazing.”

Alex said his biggest fan is his maternal great-grandmother, Ella Santoro, 91, of Manorville. That may be an understatement.

“He’s perfect,” she said. “He’s so respectful, he’s so nice. He’s a very intelligent boy. You could ask him anything and he knows all the answers. The thing of the rocks is unbelievable.”

Alex’s use of colors gives his works — each of which he creates in seven to 10 minutes — a special aura. “Everyone that’s bought them sees something that calls out to them,” his mother said.

Using Crayola and Sharpie markers, Alex paints about 50 rocks a week on average. Production increases during the summer as tourists request his work. The pieces vary in price, starting at $3 and ranging to $20, depending on size and design. He has shown and sold his pieces at five craft shows, and sells them at The Family Loft, a gift shop in Baiting Hollow; at Hallock’s Cider Mill in Laurel; and at Reeve Farm Stand in Riverhead.

“I think it’s monumental that it’s something I love that I’m turning into something successful,” Alex said.

‘It’s a real adventure’

Unpainted rocks are spread out on a long table in his basement “rock room” at home. Alex convenes with his rocks after doing his homework from Mattituck-Cutchogue Jr./Sr. High School, where he is an eighth-grader.

To gather the rocks every two weeks, he, his mother and brother, Kevin, 12, must descend 175 steps to get to a Baiting Hollow beach, “the best spot we found,” Koch, 36, said. On the way back up they carry more than 50 pounds of rocks.

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“It’s a real adventure,” his mother added. “It’s very steep.”

Birds are Alex’s favorite rock image, but he takes requests if customers ask for specific images, such as a beloved pet, a team logo or a vacation memory. His favorite bird is the common loon. “I sneak them on as many rocks as I can,” he said.

Besides being a budding entrepreneur, Alex is a budding naturalist who enjoys being outdoors — when he’s not playing soccer or basketball or running track or fulfilling his duties as vice president of the chess club or playing trumpet in the school’s jazz band or maintaining his 99.975 grade-point average.

“I’m either bird-watching or looking for some kind of animal or plant,” Alex said. “I photograph them. I like all animals. I think of myself as a sponge because I’m constantly absorbing information.”

Among his pets are two birds, three newts, a Caledonian crested gecko “with a smile on its face that makes it lovable,” a lizard, two German shepherds, a kitten, a rabbit, tropical fish, a frog and an American toad. He has two Kaiser’s spotted newts that he said “is the most endangered type on the planet.” Oh, and he also breeds caterpillars.

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His mother, a substitute teacher at her sons’ school, said she and husband Gregg Koch, 50, a stock trader, “did a lot of reading to him about nature. He’s also constantly at the library.”

‘The three P’s’

Alex is saving a lot of the money he makes from his painted rocks, but he has splurged on a bird and some frogs to add to his menagerie, as well as bird feed and a recent birthday present for his brother. Alex isn’t old enough to be on anyone’s payroll, but he does have a work ethic. “I call it the three P’s: Passion, because you can’t be good at something you don’t love,” Alex said. “Persistence: If something doesn’t go right, you have to keep doing it. Practice, to keep trying.”

When he does grow up he probably will spend less time painting rocks, choosing instead to become a veterinarian “or own my own reptile store or be a field ornithologist.”

For now, it’s all about the painted stones at North Fork Rock Art, which his father said is already generating Christmas requests. Still, there is an image Alex can’t reproduce.

“The only thing I can’t do is a human face,” he said. “It’s pretty difficult to get the personality.”