They are a hard acrylic ball, about 3" diameter, which is attached to a nylon or silk cord which is then looped and knotted to form a finger hold. This is generally wrapped around the middle finger for swinging.
Bola's are actually derrived from the hunting tool also called Bola's or Bolo's, which are hunting devices composed of three strands of rope weighted on the end. It is held by one weight and swung around and then released, wrapping itself around the legs, neck or wings of the selected target to bring it down. The Bola is named in singular because it has only one weighted end.
Bola's are constructed by taking a hard acrylic ball (such as a contact juggling ball) and using a fine drill and a vice grip, a hole is drilled through the center. On the bottom the edge of the hole should be tapered out a bit so that the knot of the cord will rest flush with the surface of the ball. The cord is then threaded through, knotted and pulled flush. It is then knotted again at the top to keep the ball from slipping up the cord. I should mention here that the cord should be at least hip length if not waist length.
Bola's are actually rhythm instruments used in conjunction with hard shoe dancing such as Tap, Gael Step, and most commonly with Flamenco. They are swung with a hard down swing to create a counter rhythm to the shoe rhythm. Use of Bola's on carpet or other buffered surface doesn't work because the plushness causes them to bounce back up, losing the rhythm. On concrete or other extremely hard surface the acrylic will break. The optimum surface for Bola's to work is a wood stage which is hollow underneath because it makes a louder noise. If this can not be accomplished then a would area resting upon the ground is also acceptible.
The patterns of Bola's is very similar to POI swinging, though the swing has a bit more force. The joy and difficulty in Bola's is to present a pleasing rhythm not only on it's own but also with use of the feet which means all the limbs going in different directions at once. A wonderful aerobic exercise and much fun, but watch out because they hurt when you miss the ground!