One, they are old and rare. Today there are only about 700 of these instruments remaining all which were made over two hundred years ago. This makes them extremely rare and extremely valuable. Because of this they need to be stored in vaults when not being used as they are frequently stolen and counterfeited.
Two, they can sell for millions. These violins are very expensive they sell for millions the highest ever paid for a Stradivarius violin at a public auction was $3,544,000. Although this may seem like a huge sum of money there have been private sales of these instruments that have vastly exceeded this amount. There are two reasons for the price being so high. The first is the sound quality these are the best sounding violins ever made and the second is basic economics, the rarer anything is the more valuable it becomes and the higher the price.
Three they are a mystery. To this day the reason for the incredible sound quality of the Stradivarius violin remains a mystery. There are many theories as two why they sound the way they do. One is that it is the glue that was used another is that it is the density of the wood used. The most popular theory is that it is the wide range of chemicals contained in the violins.
A study done in 2001 revealed that the Stradivarius violin has minute traces of borax, fluoride, chromium and iron salts in them it believed that this unique chemical composition gives them their sound.
Four they cannot be reproduced. There have been hundreds of attempts to reproduce the sound of the Stradivarius violin and almost all of them have failed. This is mainly because of the high level of craftsmanship that went into making a Stradivarius violin it is also because the materials that went into a Stradivarius violin have never truly been discovered.
Five they are controversial, not everyone believes that they are as great as they are made out to be. There have been many blind tests in which the audience and players could not find any difference between the Stradivarius violin and other similar high quality violins.