The rim of the drill bit, which is the 'cutting' surface that actually grinds it way though the rock, consists of tiny diamond particles/pieces bonded together. Hence the the term 'diamond drill'.

The bit is actually a very short tube- like fitting that srcews on to the lower end of the first drill rod. As it grinds its way through the rock, it is kept cool by water pumped down form the surface, which has thickeniong agent in it to make the water almost jelly like - (think of KY Jelly) :-). In fact,  the drill hole is actually pressurized with water, and one of many things that can cause dow-hole failure is a sudden loos of water pressure when the bit breaks through some faulted or loose rock structure, or due to equipment failure on surface. If the driller does not take the feed pressure (the hydraulic pressure pushing down on the bit) off quick enough this can burn out the bit, or in worst case scenario, actually 'weld' it to the rock. Usasally when this happens, it cannot be freed and they just put the drill in reverse and hope that rods break (unscrew) as far down the hole so they can recover most of the drill rods. What does not come out is wrtitten-off and the holes is a loss.

Failures can also occur if a rod breaks down-hole, which can be precipated by some 'rough' or jarring  drilling, when the bits pushes through some inconsistent rock, or into different types of rock. This usually only happens a rod is fatigued...

In either of the above examples of down-hole problems while drilling, if the driller is paying close attention major failures, as described above, are often avoided. However, sometimes drillers get tired or are distracted at the wrong time and then a small problem becomes bigger problems.

Bear in mind that it is crucial for the driller to be watching the water pressure gauge for indication of pressure loss the entire time the drill is pushing though the rock. Sometimes this can mean staring at a water pressure gauge for hours as the drill completes a 'run' -drills an entire rod length, at which point drilling is stopped, the core tube is pulled form the bottom of the hole, and empty core tube is dropped down the hole to latch into the lowest rod with the bit on it, and another rod is added at top and locked into the drill chuck, and the process start all over again.

And sometimes things just break for no damn good reason...

Regards,

B.