17 jewels means that there are 16 bearings holding eight axles, made of rubies. The number is odd-numbered because there is one ruby added to act as a stoppage. ("Hemmung" in german, is stoppage the right translation?)
Rubies are being used because they are hard enough not be worn out by these steel axles for years. The mineral has a Mohs hardness of 9 and is relatively cheap - only diamond has 10.