Rock Drilling Best Way??

For a hole that deep, I'd say that you'll go through 5 or 6 diamond bits to get through. Drilling my necklace pendents (about 1/8th inch thick) is a challenge, espesially on the agates, not to mention the time that it takes to one hole. If you find a drill bit long enough, you'll spend an hour or two drilling one hole. Someone else here might have different opinions, I'm probably sounding a bit pesamistic. Most of us here use a Dremel.

I've used Dremel for 1mm to 2.5 mm holes with about 4-5mm depth. I have also used Craftsman cordless drill (regular) with 7mm-11mm core drills to make 3mm-6mm tabs on the top of stones for mounting. I have also used a drill press with 28mm - 47mm core drills to cut discs out of 1/8 to 1/2 inch thick slabs. I have NOT tried deeper holes of any diameter. These tall drill bits are much more expensive.

In all these cases water was crucial. For the holes I got upto 6 holes per drill bit, but once it took two drill bits to finish a hole. Sometimes a $2 drill bit performs better than the $0.75 one, but not always. They either wear out or break. The 1mm break a lot.

The core bits last longer. For narrow ones I use to shape the top of stones, I might get 15 tops. For the larger cores, I have not had one wear out, but I only have done about 10 discs on each of my 6 core bits. Here I think more expensive sintered might last longer, but I do have a good performing braised one.

Lifting the bit to allow water to flush the hole is important. The bit does not have to come all the way out. However this might make the top of the hole a little wider than the bottom.

Another problem is spawl (a kind of blast out) on the bottom. Putting the stone on a piece of tile or brick can minimize this, but it is harder to do with a hand-held Dremel. Some even use silicone sealant to hold the stone on the brick.

Get drill bits for granite or porcelain, or lapidary bits. Like saw blades, ones designed for tile or concrete will not work well on hard materials.

For the 3" holes, consider making a core so as to remove less material.

I recommend you start mall with a 2mm hole in 1/4" thick agate. Try several types of bits, Dremel, and drill press. See what seems workable for you.

Thanks deb193... Some really good info. I'll probably start experimenting on some of these very nice granite rocks using my present Dewalt drills. But I'm guessing I'll have to get something with faster revs.

Dear Godlike docone31, My, that's an interesting technique. First thing tomorrow, I'll get some of my trusty copper wire and pipe on the job, build a well, harden the wire (I love doing that), fill up the well with some of my trusty grit and give'er hell. I figured any normal, off the rack system would be useless. Thanks so much. Much more relaxing! (Just kidding. Actually your system is over my pay-grade.)

Howdy all. I agree with all said above. My two cents worth. Drilling with diamond is not a speed contest. Running too fast just burns up cores or bits.
Granite City Tool out of Waite Park Minnesota is a good Co. that I have been doing business with for a while now and they have good people to work with you. They sell several different kinds of bits and cores. Closet to your description would be the :Blue Diamond Electroplated Drills. The small end of the spectrum is 3/16" diam. and 4" cutting depth. It will cost $58.00. It does have a 5/8" brass nut chuck end. You would have to rig it to fit into a traditional drill or drill press. The shanks and tip replacement runs $18.00. These are hollow cores for center water feed.
I always drill in a tinkers dam with a little water covering my stone. I like moulding clay for this purpose. Still your stone must not be able to move around.
Granits City also has lots of other cool products. Grits. polishes, addhesives, sealers grinding and polishing pads and heads for all kind of stone work. Check them out

WWW.Granitecitytool.com