There is a faq on diamond cutting here
http://www.encoreproducts.co.uk/advice.shtml

Peter K
As an old fart who goes back to the times when diamond was only an option if
you had NASA-type budgets, the performance of even cheapie disks is
astonishing compared with traditional abrasives. The FAQ is right in saying
that water is a GOOD THING for tile cutting machines, but in my experience
the 300 mm disks in a hand-held grinder will just power through limestone or
paving slabs without any water, as long as you have a sense of touch that
recognises when you are starting to bind because you have gone off-line. At
the smaller scale, the cheap dremel disks have diamond in a chrome? electro
plate layer which is very easy to destroy. The screwfix and aldi/lidl disks
for 115 and 300 mm grinders are *much* more robust; so far, I havn't
actually managed to wear one out (I have to admit that I don't use them all
that much, but I sliced up half a dozen 24 inch slabs last week with a 300
mm disk that I bought a couple of years ago, and I think I'm still on my
first 115 mm disk after several years too).