Hi  Mehrdad, it depends on the size distribution of what you want to grind, the chemical  composition, hardness and abrasion indices of the hearings.

It is advisable to determine the  chemical composition, grain size, hardness and abrasive indices of the ball bearings before using.

Usually balls for the ball mills are chromium-base alloy steels foundry-cast, some  times aluminum oxide balls. The angle of inclination of the ball mill  unit, rate of rotation  and time for the rotation are also critical for the particle size of the output.

Good luck. 

The best practice is to use the same material for balls and the grinding jar. However it is also common practice, in my opinion, to use "typical" bearing steel balls with a hardened steel jar. You should always take into account the contamination effect (especially when high energy or long grinding times are applied). You should also avoid using dissimilar materials (e.g. steel ball with ceramic jar).

Best regards.

Usually balls for the ball mills are chromium-base alloy steels foundry-cast, some  times aluminum oxide balls. The angle of inclination of the ball mill  unit, rate of rotation  and time for the rotation are also critical for the particle size of the output.

The correct practice requires that the material for jar and balls is the same. Another typical material to be used for grinding is tungsten carbide (WC), a very hard material. Unfortunately, the contamination effect could be present, particularly if you perform prolonged grinding at high energy. 

Above points about ball and jar material are certainly valid.  I would say, however to give it a try.  First time you do the milling remove samples at various times, and run the mill for much longer than necessary at end to check for contamination.  If you get quantifiable contamination at 1 hour of milling, for example, you can interpolate between zero at t=0 and what you find at t=60 minutes.  Then you can decide if you have acceptable level of contamination at whatever milling time is really needed.  All materials will introduce some contamination, so using the nonstandard balls doesn't introduce something new or unusual.