At what depth underground,do diamonds form???

Answer 1

Microdiamonds or nanodiamonds (created by the collision with meteors) can be found at the Earth's surface (thus, no depth at all).

It's not a matter of depth, it's a matter of high pressure (and high temperature). Via volcanoes the diamonds rise to the surface.
http://en.wikipedia.org/wiki/Volcanic_pipe

Under continental crust, diamonds form starting at depths of about 150 kilometers (90 miles), where pressure is roughly 5 gigapascals and the temperature is around 1200 degrees Celsius (2200 degrees Fahrenheit).

Answer 2

There is no certain depth ... diamonds form from carbon, if the is appropriate pressure and heat. That means, nowadays diamonds were formed millions of years ago. If they are still formed today, it happens probably close to the edge of the solid earth's mantle, only there is enough heat an pressure.
Over time (millions of years), those diamonds will move to higher layers due to the drift of the continental plates or come up with vulcanoes or similar ...

Answer 3

Likely more than 150 kilometers down in the erath's mantle.