what temperature-pressure conditions are required fopr this?
High pressure, high temperature.
GE diamond project
Hall achieved the first commercially successful synthesis of diamond on December 16, 1954, and this was announced on February 15, 1955. His breakthrough was using a "belt" press, which was capable of producing pressures above and temperatures above 
when was the first man made diamond made 1954 what are the materials required to make this? Pure carbon, the belt press, the cubic press and the split-sphere (BARS) press.
1400 °C 1400 °C High pressure and high temperature
Synthetic diamond Synthetic diamond Synthetic diamond
High pressure, high temperature High pressure, high temperature Manufacturing technologies
In the HPHT method, there are three main press designs used to supply the pressure and temperature necessary to produce synthetic diamond: the belt press, the cubic press and the split-sphere (BARS) press. Diamond seeds are placed at the bottom of the press.
The internal part of press is heated above 1400 °C and melts the solvent metal. The molten metal dissolves the high purity carbon source, which is then transported to the small diamond seeds and precipitates, forming a large synthetic diamond. The internal part of press is heated above 1400 °C and melts the solvent metal. The molten metal dissolves the high purity carbon source, which is then transported to the small diamond seeds and precipitates, forming a large synthetic diamond. The original method uses high pressure and high temperature (HPHT) and is still widely used because of its relatively low cost. 
wiki:7472818
Synthetic diamond [SEP] GE diamond project
His breakthrough was using a "belt" press, which was capable of producing pressures above and temperatures above . The press used a pyrophyllite container in which graphite was dissolved within molten nickel, cobalt or iron. Those metals acted as a "solvent-catalyst", which both dissolved carbon and accelerated its conversion into diamond. The largest diamond he produced was across; it was too small and visually imperfect for jewelry, but usable in industrial abrasives. Hall's co-workers were able to replicate his work, and the discovery was published in the major journal "Nature". He was the first person to grow a synthetic diamond with a reproducible, verifiable and well-documented process.