Silestone is a hard quartz material and requires drill bits that are harder than the quartz material. Even with the proper drill bit, heat will build up in the drilling area, and as the heat will be concentrated, the Silestone can crack and ruin an expensive countertop or tile surface. To control heat, you need to lubricate the drilling area with water. Water will draw the heat away from the tip of the drill bit and reduce the friction created between the tip of the drill bit and the surface of the Silestone.
Cover the area where you need to drill the hole through the Silestone with masking tape, and mark the location of the hole on the masking tape with the permanent marker.
Ball up the modeling clay to the size of a baseball and roll the ball of modeling clay on the Silestone to make a rope roughly 1/2 inch in diameter and 14 inches long.
Press the modeling clay rope onto the Silestone to form a sealed circle around the location of the marked hole. The diameter of the modeling clay circle should be at least three times the diameter of the hole you will be drilling.
Fill the modeling clay circle with water.
Secure the diamond drill bit into the chuck of the variable-speed drill motor, and put on your safety glasses.
Place the empty one-gallon plastic bucket under the Silestone, aligned with the location where you will drill the hole.
Align the diamond drill bit with the location you marked on the masking tape, and pull the trigger of the drill motor to drill the hole through the Silestone.
Remove the modeling clay circle from the surface of the Silestone, pull the masking tape from around the drilled hole, and wipe the Silestone with a dry rag to remove excess water from the area around the hole.