Beware!!!
A diamond bit is expensive and will chip VERY easy. Once its chipped it is junk. It does give the best quality cut though.
It can be resharpened but you have to find a place that specializes in diamond resharpening.
A carbide bit is much much cheaper and will last a long time with care. Done right, a carbide bit can yield almost equal results to a diamond. The trick is RPM. A 4 cell pack (4.6 volts) spins the come at too high an RPM thus dulling a carbide bit quickly.
If you can get a variable power supply and set it a 2 volts (like TSR6 says) A carbide bit will cut well and last a long time. You can sharpen a carbide bit at any machine shop or Automotive repair shop.
IMO, if your new at this, don't waste your money on a diamond.