Drilling carnelian

A friend of my mother’s (and I know, as I type that, that really I
should have known) asked me to make some stones which she and her
late husband collected together into jewellery.

These are all carnelians and agates found by the se, and ready
polished. I’m by no means a lapidary but I have drilled stones
before and agreed without any qualms. But drilling these is proving
to be a ridiculously slow and painful process; I’m using solid
cylindrical bits at about 10,000 rpm. Is there anything else I could
be doing, or is agate really that hard?

sorry to be so dense.


If your drill will go faster then use a higher speed and make sure
that you flush the hole really well. Drilling under water is best,it
cleans the hole and cools the bit which stops the diamonds becoming
glazed over. Apply light pressure and then release in pulses of a few
seconds each.


Thank you Nick - I do always drill under water if I’m
drilling/grinding stone or glass, and I did used to use a faster
speed. But the manufacturer’s site (for the drill-bits) recommends a
much slower speed to prolong the life of the drill-bit, so that’s
what I’ve been doing. I’ll give it a few more rpm and see what

thank you again!

Hi Sophie,

I had a job like this and I used small core drills, if you think
about it the centre of the drill is not moving much on a solid drill,
so the drill just spins on that spot.

I think I got mine from Rio Grande, I get an order from them a
couple of times a year (I am also in the UK). They go down to about
2mm, I still found it a slow process

regards Tim Blades.

Tim, do you mean hollow core? I might bear that in mind for next
time. 2mm is a bit big for what I’m doing, sadly - and I have now
passed a message on to say I think the original plan is not going to
work. I can’t believe how hard this stuff is! I’ve happily drilled
agatised coral before and expected it to be similar. Not.

I get diamond bits from here, I thoroughly recommend them. Their
hollow core bits only go down to 3mm though.

thanks for the help!

Hi Sophie,

Rio do even smaller than I remember, on their website

there are two products,

349041 individual sizes down to 1.25mm

349016 a set down to 1.0mm

I don’t know how long they would last but they exist.

I believe that in India they mount a tiny diamond chip in a
hypodermic needle and drill with that.

regards Tim Blades.