The 300-year-old Stradivarius violin that was taken from the Milwaukee Symphony Orchestra's concertmaster in an armed robbery is pictured in Milwaukee, Wisconsin in this February 6, 2014 handout photo at a news conference after it was recovered. Reuters/Jon D. Riemann/Milwaukee Police/Handout

On Thursday, a stolen Stradivarius violin worth an estimated $5 million was recovered by Milwaukee police. The (almost) priceless instruments are common targets for thieves, but what makes Stradivarius violins so special?

To be deemed a true Stradivarius, an instrument must come from the workshop of the Stradivari family during the 17th and 18th centuries. The patriarch of that family, Antonio Stadivari -- an acclaimed master of stringed-instrument-making -- taught others masters, whose instruments can go for a pretty penny as well.

Only an estimated 650 Stradavarius instruments exist around the world. Most are in the multi-million dollar range. The most expensive (and also the most expensive instrument ever sold at auction) went for nearly $16 million back in 2011. The proceeds benefitted tsunami victims in Japan.

Many Stradavarius violins, like the one stolen in Milwaukee, are valued so high not just for their age and craftsmanship but also for their impressive line of owners. The Milwakee Strad is known as the Lipinski Stradivarius because it was once owned by the 18th century virtuoso Karol Lipinski. It even has its own Wikipedia page.

Back in 2010, a thief stole a Strad worth nearly $2 million from a violinist while she grabbed a sandwich. The thief had no idea what he had stolen and tried to sell it to bus driver for £100. Just the bow itself was worth $100,000.

Philippe Quint left his $4 million Stradivarius in a Newark, N.J., taxi cab after a flight home from Dallas. The cab driver, Mohammed Khalil, returned the violin within hours and Quint played a private concert for the Newark Taxi Cab Association to show his gratitude. He also gave Mr. Khalil a $100 tip for the safe return of the instrument.

Not all Stradivarius violins make it back to their rightful owners, though. In 1995, a Stradivarius worth $3.5 million dollars was stolen from a dying violinist’s apartment. It’s known as the Davidoff-Morini Stradivarius and it has yet to be found. The Davidoff-Morini violin sits on the FBI’s top 10 art crimes list along with pieces by Van Gogh and Cézzane and artifacts stolen in war-torn Iraq in 2003.

How do Stradivarius's compare to other instruments in price? They blow them out of the water. The most expensive guitar ever sold was Eric Clapton's famous "Blackie" Fender Stratocaster that went for just under $1 million. Even an entire auction of Clapton pieces went for $2.15 million in 2011, lower than four out of the five single Stradivarius violins mentioned in this article. All things considered, $2.15 million for a load of equipment less than 100 years old and far cheaper at the time of purchase isn't bad.

Ironically, there’s no real market for stolen Stradivarius violins. So many are well-documented and known that they can’t “disappear” in a backdoor deal. We don’t know what the Milwaukee thieves had planned for the stolen Lipinski Strad, but we do know it’s sure better off back in the hands of the talented Mr. Almond.