Idar-Oberstein, two German towns hyphenated into one, is the gem capital of Europe. Nestled in a rocky gorge carved out by the Nahe River, it is world famous for the precious stones that once were mined, and are still cut, polished and set here.

But gems are only half of this glittering town's claim to fame. At the end of the last century, a quirk of fate linked the famous gemstones to a gastronomic wonder -- spiessbraten -- as surely as Idar was linked to Oberstein.

Spiessbraten is wood-grilled meat -- thick, juicy, flame-charred slabs of beef or pork, dark and crisp-edged, redolent of onion and garlic -- and a flavor you can't pinpoint that is utterly seductive. Pungent, rich and delicious, spiessbraten is a culinary gem among the gems.

Spiessbraten was brought to the town around the turn of the century by German prospectors returning home from Brazil, where they'd gone in search of new gems to conquer. They had fond memories of the meat they had enjoyed there, roasted crisp and smoky over a wood camp fire, and they soon devised a method to duplicate the recipe in Idar-Oberstein. One heavenly bite and their families were hooked. The prospectors had brought back yet another gem.

Today, spiessbraten is as ubiquitous in Idar-Oberstein as the famous gemstones. Every restaurant offers it, accompanied by crusty rye bread and the requisite salad of grated radish -- the white, foot-long, 2-inch-thick kind -- dressed with vinegar and cream.

But the ultimate treat is to eat spiessbraten in a private home where it is prepared by the masters, the men of Idar-Oberstein. The art of preparation is handed down from father to son, and is considered a rite of passage to manhood.

Literally, spiessbraten means "spit cooking," which is how it was done in the wilds of Brazil. But necessity yielded to art when the citizens of Idar-Oberstein adapted the recipe to the medieval, open-fire grill with which every household seems to be equipped: a round grid suspended horizontally by chains from a six-foot tripod. The meat revolves slowly through the flames of a wood (never charcoal) fire. As the fire dies, the chain is lowered, keeping the meat always brushing the flame. It's a far cry from throwing a steak on the barbecue, and the flavor it produces is matchless.

But there is more to the mystery of spiessbraten's flavor than the way it's cooked. And yet when I asked old friends the secret of the marinade, it seemed too simple: salt, pepper, onions and garlic. Granted, the recipe couldn't be too complex since it was originally camp cookery, but surely some exotic leaf or root, some special secret, flavored this meat.

Eagle-eyed, I watched the meat being prepared for marinating, watched it being cooked. Later I repeated the process at home. Each time the flavor was glorious, and I had to acknowledge that there was no sleight of hand, no mystery spice involved.

The secret, I now think, is salt. I had never given salt credit for much more than making you thirsty and holding the ladder for the more glamorous herbs and spices, but spiessbraten has taught me that salt itself imparts a special pungency: It draws flavor from the onions and marries it to the meat. What still puzzles is that the salt does not draw juice out of the meat as I had assumed it would. And you don't feel thirsty after eating spiessbraten.

Of course the flame cooking adds its own magic. Idar-Obersteinians are very precise about which woods must be used for cooking: hard, bark-free woods with little sap. I would be drummed out of Idar-Oberstein for the following claim, but I will make it anyway: I have grilled spiessbraten over a charcoal barbecue. I have even grilled it in my oven broiler. Of course it wasn't quite as glorious as the real thing. But while not the Olympian original, it made you smack your lips and surrender your dignity fighting for the last succulent piece off the platter.

So, go to Idar-Oberstein to eat the ultimate spiessbraten. But while you're there, don't neglect the other wonders.

The streets are lined with shops whose windows glitter with gemstones -- polished slices of agate; jagged, glinting chunks of amethyst; gleaming crystal formations. Every example of the jeweler's art is on display in countless necklaces, brooches, rings, pendants.

In bright windows, delicate flower bouquets of carved gems twinkle. There are tiny jewel plants, dainty gold-wire trees with leaves of jewels -- blue, lavender, pink, pale and dark green. There are fat bunches of agate or jade grapes. There are clocks and ashtrays of gemstones; polished stone handles of every color adorn letter openers, bottle stoppers, knives and forks.

Even the doorknobs here are immense hunks of polished agate, jade or tiger's-eye. Simple restaurants and tea shops boast Tiffany-style lampshades of gold, orange, brown or ivory agate. It's impossible to forget that Idar-Oberstein's reason for being is stones.

Plan to spend a long, wonderful day here, for Idar-Oberstein offers a wealth of things to see and do:

The Steinkaulenberg mine, which was worked in Roman times, is the oldest jewel mine in Europe that is still accessible. The moisture-slicked walls of its dark tunnels glitter with nodules of agate, jasper and carnelian. As you walk through, the guide's flashlight picks up the eerie gleam of hollow amethyst and rock crystal druses.

At the old Weiherschleife grinding mill, whose water used to turn the great sandstone wheels that ground and polished stones, workers lay on tilted trestles laboriously pressing rough stones against the grinding wheels to shape, then to polish. Go from there to one of the many gem shops, where stonecutters demonstrate modern cutting and polishing methods. Of course, the shops offer treasures for every taste and budget, from simple agate key chains to jeweled tiaras.

The Felsenkirche, hewed into the sheer rock face above Idar-Oberstein, is as dramatic a church as you will ever see. It's said two brothers quarreled at the site over a fair damsel; one hurled the other down the sheer cliff, and later atoned for his sin by building the church at the scene of the crime.

The town's two gem museums are unique in Europe. The Idar-Oberstein Museum, privately owned, is on the Hauptstrasse, the main street, and exhibits in dazzling profusion every stone known to mankind. It also displays massive hunks of amethyst and quartz that a big man would strain to put his arms around. There are superbly delicate cameos, ancient seals whose elegant, minute figures of men and animals must have ruined the eyesight of countless artists and whose beauty will hold you enchanted for hours.

The Deutsches Edelstein Museum on Mainzerstrasse, which includes a diamond and precious stone exchange, is equally splendid. At this one, municipally owned, the focus is on artworks in gemstone -- huge shimmering bouquets of flowers, stems, leaves and blooms all of delicately carved gems. There are figures in gemstones of men and birds and animals, and huge, near-transparent agate flakes whose luminous whorls depict eerie, phantom landscapes.

But the time comes when you have feasted your eye and spirit, when your mind brims with the lore of stones and cutting. You've hit the shops, explored the ruined castles and climbed to the church in the rock. It is time for spiessbraten. Only then can you leave Idar-Oberstein -- with a dazzled spirit, a bursting shopping bag, a flattened purse and a gently rounded stomach that gurgles in contentment.

For more information on Idar-Oberstein, contact the German National Tourist Office, 747 Third Ave., New York, N.Y. 10017, (212) 308-3300.

Polly Clingerman is a Washington-based food and travel writer currently living in Europe.

Traditionally, spiessbraten must be grilled over the flames of a wood fire. However, you can make a delicious version over charcoal or under your oven broiler.

SPIESSBRATEN (4 servings)

4 steaks of beef or pork, boneless, 1 1/4 to 1 1/2 inches thick, about 12 to 16 ounces each. (If using pork, cut from the leg or any traditional beef steak cut)

1 to 2 teaspoons salt

White pepper, freshly ground

2 cloves garlic, sliced thin

1 to 2 medium onions, sliced as thinly as you can

The night before (or early on the morning you plan to cook), lay out your steaks on a work surface. Push the thin garlic slices into every natural separation you can make with your fingers. Use about 1/2 clove of garlic per steak.

Because salt is used not to enhance other flavors, but to impart a specific flavor, give the process proper attention. For a steak that is 1 1/2 inches thick, 3 1/2 inches wide and 6 1/3 inches long, use about 1/8 teaspoon salt per side.

Put the salt in a small saucer,then pick it up with your fingertips and drizzle it over the meat slowly, in a very fine stream. Make a very thin, even film that covers every bit of the steak's surface.

Next, grind on a thin, even layer of pepper. Again, cover every bit of surface.

In a non-corrodible bowl or flat dish, place a layer of paper-thin onion slices that will completely cover the underside of the meat. Lay the steaks on this.

Completely cover the meat surface with onions. If you are preparing several steaks, you can stack one layer atop another -- just be sure that each layer of meat is always totally covered by onion slices.

Cover the dish tightly and let it marinate for at least eight hours -- overnight is better. Keep refrigerated.

Allow steaks to reach room temperature before cooking. Meanwhile, bring your charcoal fire or the broiler to a very high temperature.

Scrape the onions off the meat and grill or broil it two to three inches from the heat source. The meat should be brown and crusty on the outside, juicy pink inside. Grilling takes about four to six minutes per side.

If you are using a traditional spiessbraten grill, pile the wood for your fire -- only bark-free, hard, sapless wood such as beech, maple and birch -- on the ground directly under it. Set it alight. When the flames are very high, orange-yellow and hot, place the meat on the wheel and give the chain a gentle push to set it revolving slowly through the top of the flames. Keep the grill moving. The object is to sear the outside of the meat quickly. When the bottoms of the steaks are seared, turn the meat and sear the second side. This should take a total of 10 minutes.

Add no more wood to the fire. The meat should start on a very hot fire and finish in diminishing flames. As the fire lowers, lower the chain so that the grill continues to slip through or graze the top of the flames. Grill the meat about 10 minutes more, turning now and then. A total of about 20 minutes will produce a rare steak. Leave it about five minutes more if you prefer it well-done.

Serve immediately. Fat slabs of rye bread and a salad of giant white radishes are the traditional accompaniments. Grate the radishes on the big holes of your grater and dress them with whipping cream mixed with a little vinegar, salt and pepper. A crisp green salad also goes well.

-- Polly Clingerman