Drills used on wood or metal have a sharp metal tip or teeth which cut into the material. Diamond drill bits come in many different sizes but are of two basic types - blunt nose bits and core drill bits. Blunt nose bits are solid at the tip and have diamonds coated on the tip and on the sides of the tip. This type of bit drills a solid hole by grinding a complete hole the size of the tip. Core drill bits however, are hollow at the tip, much like an upside down drinking glass.
Core drill bits grind a circle at the edge of the tip rather than the full diameter of the tip.
Since core drills are only drilling out a portion of the resulting hole, they tend to drill somewhat faster than blunt nose drills. However, drilling in glass, marble, or ceramic is a slow process compared to say wood. Diamond drills should be used at slow speeds - approximately 300 to 600 rpm. The lubrication reduces heat build-up and heat fractures in the material. If properly lubricated, the dust from the cut should be of the consistancy up a moist paste - say the consistancy of Elmers Glue.