The size of the diamonds used in surface-set drill bits are normally described as the number of stones per carat (SPC). The general rule for selecting the appropriate diamond size is that the harder the rock, the smaller the diamond. For example, a hard, relatively non-abrasive granite formation in the 6 to 7Moh’ss hardness range would probably be best drilled with a 100 SPC surface-set diamond bit while a soft, abrasive limestone formation in the 4 Moh’s hardness range might be best drilled by a 10 SPC surface-set diamond bit.

As the diamonds that are used in surface-set bits are naturally occurring, they fall into a broad range of sizes. Each of the SPC sizes that are referred to in describing IDP’s surface-set drill bits are actually nominal values that represent a set range of SPC sizes and are defined in the following table.