Underneath Fifth Avenue in Midtown Manhattan, past the showroom of gems, minerals, and fossils worth tens of thousands of dollars, through a locked door, down some stairs, behind a half-foot-thick steel vault door, lies Astro Gallery of Gems' crown jewels.

First, Dennis Tanjeloff, the CEO and grandson of the late founder, unwraps a palm-size chunk of crystallized leaf gold, which is wrapped around quartz and was found in a cave in Nevada. He says it's worth about $4.5 million, and no pictures please--he wouldn't want to cheapen a piece he has reserved for one of his billionaire clients. Tanjeloff next opens a tiny box to reveal a 2-inch long marine green crystal, a mineral named Hiddenite, which only comes from certain mines in North Carolina, worth about $750,000. Another prized specimen is a 400-karat, gem-quality aquamarine from Pakistan resting on a piece of quartz, which sells for $650,000.

Astro Gallery of Gems, which sells gems, minerals, fossils, and rare earth crystals and curates collections for museums, has been a family-owned business since founder Julio Tanjeloff opened up a wholesale gem business in Manhattan in 1961. Sitting on a couch next to a glass table that holds a more than 15 million-year-old Moroccan crane skeleton, Dennis talks about how his grandfather got into selling precious gems and minerals.

In the early 1950s, he says, Julio was in the real estate business in Argentina when he bought a few properties along the Rio Negro in Patagonia. The people who ran the manganese mines on the land taught him the ins-and-outs of semiprecious stones and the import-export business of gemstones. Julio began exporting to Germany and Europe, and it became his life's passion.

Near the end of that decade, the government began to seize Tanjeloff's property, along with other citizens' land, and Argentina erupted in violence. Julio fled to New York and opened a wholesale business. He eventually built a retail gallery and Astro Gallery of Gems was born.

Breeding a CEO

In the 1970s, Julio owned 10 to 15 mines all over South America, bought ivory and cinnabar in China, and traded gems from the mountains of Pakistan and Afghanistan. He regularly traveled to Morocco and India as well, often with Dennis, who was then a young boy, accompanying him.

"I was bred into this like a horse. By the age of four, my grandfather was taking me to all around the world. By osmosis and through being around the business all the time, even though I thought I was bored, it was all coming in," Dennis tells Inc. "The information, the people, the cultures--all the aspects of doing business internationally, flooded through me into my teens."

By the time Dennis was 20 he had already been to mines in more than 30 countries and started to travel on his own to buy minerals and gems for the business. In 1988, Julio died and Edgar, Julio's son and Dennis' father, took over Astro. Dennis worked alongside Edgar until 1996, when Dennis became president and CEO.

The business Julio had built was expansive--he opened over 20 Astro Gallery retail outlets in department stores across the country, acquired other stores and wholesalers, and launched Mineral Digest magazine. He had also built an impressive portfolio of mine and land rights, but as the veins of precious minerals and gem-crystals dried up, the company sold them off. Dennis and his brother Marc, who is Astro's vice president, recently sold a titanite mine in Tanzania, the last mine the company owned for the time being.

Still, the brothers are constantly traveling around the world to find specimens for their deepest-pocketed clients. "I could get a phone call from Peru, Uganda, or Australia," Dennis says. "We're always chasing the new minerals and discoveries as well as continuing to purchase from mines that are still producing." They recently got back from Uruguay, where they bought a supply of calcites that formed on top of amethyst. One piece will sell for $200,000. At the end of May, they will go Hunan and Changsha in China to check out the sulfide mines.

Every year, Astro Gallery of Gems buys and sells thousands of pieces, with inventory valued at a total of up to $20 million annually. This year they expect $11 million in revenue, a 20 percent increase over 2014, thanks to aggressive purchasing, the quality of the inventory, a strong partnership between their children's outpost Astro Kids and toy store chains, and the growth of their e-commerce arm.

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Digital rock sales are up

In 1998, Dennis opened Astro's eBay store. Now, Astro's own e-commerce site brings in 30 percent of the company's revenue. "Our e-commerce site sells all types of specimens. It could be a $20 fossil for a kid in Kansas to a $5,000 coral statue to decorate a house in Miami," he says.

Below the retail showroom is where you can watch the inner workings of Astro's online business. In a room full of metal shelves holding gems, geodes, and precious minerals and gem crystals, about half a dozen employees are at work washing specimens, packing them up, and accepting deliveries.

One employee tends to giant pieces of coral just delivered from the Philippines, while another wraps framed insects to a buyer in China. A geode from Uruguay and massive pieces of apophyllite and pink stilbite from India, which Tanjeloff says "formed millions of years ago in volcanic gas tubes," will be shipped to a new hotel lobby in Dubai.

Staying ahead

Tanjeloff walks around the store, animatedly holding forth on the pieces of meteorites for sale. He points to rubies and emeralds and tourmaline and azurite and explains how the minerals were formed millions of years ago during a seismic event. He mentions some of Astro's famous clients over the years, a list that includes John Lennon and Salvador Dali. He describes some of the one-of-a-kind pieces that have come through Astro's showroom and vault, like a thousand-pound piece of antimony stibnite from a famous pocket in China, which the company donated to the Museum of Natural History. 

But like any retail business, products come and go. Once he sells the prehistoric bear skeleton, or the fossilized lily, he'll have to be ready with some other rare pieces to keep his customers happy. "It can take 10 years to finish a client's collection," he says. "The relationship with the client is like a marriage, it's very sophisticated--you become friendly with the family and go to their home."

Once a good mine starts to yield low-grade minerals, you have to find the next one. But for three generations, the Tanjeloffs have always been able to stay ahead of the market.

"The inventory changes constantly as Mother Nature produces," Tanjeloff says. "As these things sell, my job is to replace them with something else--[whether it's] a 2,000-pound piece of petrified wood table from Arizona or a 600-pound amethyst geode."