Diamond drilling refers to the technology whereby rock is cut by a bit that is impregnated with diamond chips. The bit is typically hollow and as it turns, the diamond chips cuts away a circular portion of the rock leaving a core of rock material which is pushed up into the core barrel of the drill. When the core barrel is full a wireline is attached to the top of it and it is pulled away from the bottom of the drill hole and pulled to the surface. At the surface, the core of rock that has been cut is extracted from the core barrel and put into a core box for logging and subsequent assaying.

The process sounds deceptively simple but there is a significant amount of technology in diamond drilling and, perhaps more importantly, a lot of art in the business. A bit that is suitable to one rock type may not work in a different rock type and the type of fluid which is used to lubricate and cool the bit while keeping the hole open is an important parameter in maximizing the recovery of core from the hole. If the rock is highly fractured it will be difficult to obtain an acceptable core recovery unless mitigative measures are taken.

The power train which rotates the bit and applies pressure to the bit face is also important to efficient operation. Recent changes in design have resulted in most drills going to an all hydraulic operation which allows much better control over rotation speeds and downhole pressures. Once again all the design changes are made to increase penetration rates while maintaining high core recoveries.

Diamond drills come in a variety of sizes and generally, the larger the drill, the deeper it is capable of drilling. As many drill sites are very remote there is an increasing demand for smaller, man-portable drills that can be carried into difficult sites. With all-hydraulic mechanisms, these drills are capable of drilling up to 450 meters deep. Bit sizes can also vary and the most popular mineral exploration sizes are BQ (36.3 mm), NQ (47.6 mm) and HQ (63.5 mm). The smaller the bit size the deeper the drill can penetrate but smaller core sizes usually result in lower core recoveries therefore there is a tradeoff that must be made in selecting a core size.

Diamond drilling is generally the most expensive way to obtain mineral samples for assaying but it is also the most direct and accurate way of knowing what is below the surface of the ground. Using specialized instruments it is also possible to identify the direction of predominant rock structures, fractures and joints which is useful in understanding the geotechnical capacity of the rock.