What is the rattle of a rattlesnake made of?
Snake scale
The most distinctive modification of the snake scale is the "rattle" of rattlesnakes, such as those of the genera "Crotalus" and "Sistrurus". The rattle is made up of a series of loosely linked, interlocking chambers that when shaken, vibrate against one another to create the warning signal of a rattlesnake. Only the bottom is firmly attached to the tip of the tail. 
At birth, a rattlesnake hatchling has only a small button or 'primordial rattle' which is firmly attached to the tip of the tail. The first segment is added when the hatchling sheds its skin for the first time. A new section is added each time the skin is shed until a rattle is formed. The rattle grows as the snake ages but segments are also prone to breaking off and hence the length of a rattle is not a reliable indicator of the age of a snake.
Rattleback
A rattleback may also be known as a "anagyre", "celt", "Celtic stone", "rebellious celt", "rattlerock", "spin bar", "Robinson Reverser", "wobble stone" (or "wobblestone") and by the product names "ARK", "Bizzaro Swirl", "RATTLEBACK", "Space Pet" and "Space Toy".