PROVING HIS METAL: Detectorist demonstrates the art of the beach dig in La Jolla

“Here we go,” says Mark Ruby.

The 65-year-old retired auto-shop owner from Allied Gardens has discovered yet another target in the sand with his metal detector.

He reads the numbers on his view screen, which tell him the metal content and conductivity, letting him guess what it is. “It’s reading kind of high for stainless,” he says.

Ruby digs his sand scoop in, shakes it out vigorously and voila! A silver ring reveals itself.

“That’s the stuff we like to find,” he says as he searches it for maker’s, purity or any other identifying marks. (There are none, so it gets tossed in the “finder’s keepers” pile.)

Ruby — who goes digging three times a week either on his own or with other members of the San Diego Coinshooters detecting club — has come to La Jolla Shores to teach me how to perform this magic. Recently, I bought a used metal detector at a garage sale. (After all, I’m never going to strike gold as a journalist.) But I have no idea how to use it.

The metal detector — invented in 1874 by Gustave Trouve of Paris to locate bullets lodged in gunshot victims — employs an electric coil that passes a magnetic wave into the ground. When it hits typical sand or rocks, it doesn’t react. However, when it strikes metal, it creates an electrical current that the detector is tuned to pick up.

Ruby bought his first metal detector for $60 at age 17, after seeing an ad in a magazine promising treasure to be found. As a Navy brat who grew up “everywhere,” he says he hunted with it on bases where his father was stationed.

“As a kid with a coin collection, it was getting harder all the time to find decent coins in circulation,” he says. “The early days were great. You had hardly anybody detecting back then.” (According to a 2014 estimate by the Task Force for Detecting Rights Foundation, there are now more than 300,000 detectorists in the U.S. There’s even a British comedy series about the hobbyists airing on Netflix called “Detectorists.”)

Ruby’s current machine is a Minelab E-TRAC ($1,549), constructed of plastic to be super light. Even his scooper costs $500, because it’s carbon fiber, which won’t set off his detector.

Mine cost $20, because it’s made of metal and was manufactured in 1980.

Ruby grabs it and stares at it like an archeologist would — like an archeologist would a relic, not a tool. (“Where’s the wind-up key?” he jokes.)

Ruby slaps down a quarter and I slap on my headphones. My detector refuses to bwoop.

“It’s probably set up where you’re not getting all the low conductors,” he says, which would be my thoughts exactly if I knew what they meant.

After more tinkering, my machine bwoops at the quarter and Ruby pronounces me ready for our dig. He throws out some tips along the way, which I will scatter throughout this article like silver rings for you to find.

TIP 1: Dig during a low tide or after a big storm.

“Gold is heavy and doesn’t move,” Ruby says. “Usually, it gets deeper in the sand. It takes a real big storm to move something heavy like that — and only if it’s moving the sand along with it.”

OK, so we’ve already broken our first rule because it’s neither a low tide nor after a big storm. But Ruby says there are still “fresh droppings” calling our names out there. After all, he says, we’re on one of the biggest and most affluent beaches in San Diego. And affluent people wear nice jewelry that they sometimes lose on the beach. Just up the sand four years ago, Ruby says, he found a $30,000 gold ring.

Bwoop-bwoop-bwoop!

My machine goes off big time, just seconds after I walk it around live for the first time.

This is probably a good place to mention that, in 2009, Scottish metal detectorist Terry Herbert asked a farmer friend if he could scan his field to find a few coins. Within days, he discovered the largest collection of 7th-century gold and silver metalwork ever found. Its worth? More than $5 million!

Nope, I merely detected the metallic sand scoop Ruby lent me, which my gait was swinging too close to the detector. (At least I know if there’s a sand scoop buried somewhere out there, I will detect it.)

TIP 2: Drag your sand scoop behind you, to avoid detecting it and to trace out a grid of lines so you don’t go over the same area twice.

Ruby conducts his digs with the military precision of his Navy upbringing, swinging his detector’s coil back and forth in a way that perfectly maximizes his searched area.

Not only is he getting more bwoops than me, he’s even predicting what he’ll find before he digs (a quarter, a nickel, a pull tab, a bottle cap). He’s like Babe Ruth pointing to the left-field fence!

The pull tabs and bottle caps Ruby digs for anyway because they register similarly to gold rings, and you can’t tell for sure until you dig.

“If you want to find gold, you’re going to be digging lots of foil, nickels, pull tabs and bottle caps,” Ruby says. “They can’t discriminate out exact metals. They do it by conductivity.”

Turns out, Ruby detects another pull-tab. He moves it from his scoop to his pouch anyway, because one of a metal-detectorist’s responsibilities is cleaning up trash.

“It’s the right thing to do and it makes people respect us more,” Ruby says, “so they don’t think we’re just bums.”

TIP 3: Locate and detect on what Ruby calls “the towel line.”

One person lays a towel down, then others, by dint of human nature, assume that the first person knows where towels are supposed to go. Eventually, a long single line of towels forms. The next morning, it becomes a long line of objects that fell off of towels.

“Most of your good targets will be things that people lose, not Spanish doubloons washing up onshore,” Ruby says.

Some jewelry does occasionally wash up on shore, Ruby says, but it’s not likely to be from a 17th-century shipwreck. Rather, it’s because cold water shrinks the fingers of swimmers and other revelers, relieving many of their shiny adornments.

Hmm. It occurs to me that we’re preying off people’s misfortune here.

“To a point, yeah, sure,” Ruby replies.

But Ruby’s not profiting off it personally. He a member of Ring Finders, an organization dedicated to returning lost valuables to people for free.

“I leave it up to them if they want to leave a reward,” Ruby says. “I don’t ask for it, but I’m happy to accept it.”

That $30,000 ring he found? It was a desperate Ring Finders call from a visiting Bay Area couple.

“She came down here for a bachelorette party and decided to rent kayaks and paddle over to the Cove,” Ruby says. “When she came back, she got tumbled by the surf and everything she had in the kayak ended up out in the water, including her engagement ring.”

The next day, Ruby scoured the ocean bottom for eight hours with his detector (which, unlike mine, is waterproof).

“I found five other rings but didn’t find hers,” he says. “But I found it about a month later, when we had a real super low tide and glassy conditions like a lake.” (See? That’s why that was our TIP 1.)

A woman approaches, perfectly on cue. She identifies herself as Elena Stasevich, a tourist from Russia. Two weeks ago, Stasevich says, she lost a gold Madonna-and-baby medallion from her chain near Lifeguard Station No. 33.

Retirement will have to wait. We detectorists are now on a mission.

The problem is, after more than an hour scouring the sand, I have yet to detect a single thing. So I randomly flick switches and …Bwoop-bwoop-bwoop!

My machine starts working almost immediately. Someone’s about to get her Madonna and baby medallion back! I dig and dig and …

Nope, just a rusty nail.

Ruby explains that I’m near a makeshift (illegal) fire pit, where someone probably burned a palette for firewood. He could tell by the charred fragments of wood all around me that I didn’t notice. He says that modern machines can be set to discriminate out signals from iron.

Boy, the beaches of 1980 must have been completely devoid of rusty nails.

TIP 4: Don’t detect over makeshift (illegal) fire pits.

Oops, I didn’t weave Ruby’s tips in very evenly. I’m almost done and I’ve got a ton left. Here they are clustered all together like the pull tabs Ruby’s been finding…

TIP 5: Detect over the high-tide watermark. It’s where the waves push everything up.

TIP 6: Detect over displaced sand — any that’s moved from one beach area to another.

TIP 7: When you find one coin, work the area. There may be more.

TIP 8: Don’t get too close to beachgoers. Some don’t like that.

TIP 9: Fill all your dig holes back in. It’s just nice.

Ruby and I retire to a table in Kellogg Park to, as he calls it, “play show and tell.” He pulls out of his pack today’s booty. It contains about 20 pull-tabs. Other than how disappointing it is to find these instead of gold rings, there’s something that bugs Ruby about them.

“Why do people pull these off of cans?” he asks. “They’re designed to stay on them!” (Mind blown. I hadn’t even noticed that!)

The Club donates the pull-tabs to Ronald McDonald House. Since they’re aluminum, the charity makes money selling it. “It’s the right thing to do,” Ruby says. “Plus, we get tons of junk off the beach every year like this.”

If this is Ruby’s normal daily bounty, however, it occurs to me how long he would need to work just to earn his $2,000 equipment investment back. (Upon closer inspection, Ruby declares the silver ring worth only about $10.)

When asked how much money he’s made metal-detecting since 1970, Ruby replies: “That’s hard to say because your costs usually outweigh what you’re bringing in.”

At my rate of one rusty nail per day, I would need to work even longer to earn back my $20 than Ruby would for his $2,000.

“If you’re looking to get into this hobby to get rich quick,” Ruby says, “you can forget about that.”