Debris in radiator..opinions..

Ok motor has 300 (total)miles fairly trouble free..3 oil changes with clean oil in and good color out. Temperature is 180-190 consistant and runs without a hitch. I have noticed brown specs in the coolant(new coolant) on the surface..when inspected with a light the coolant does not display the "rainbow" of colors that oil in the coolant shows on the surface, and the coolant itself is perfect in color. My radiator is older and yes has some buildup, but I am curious as to what are signs of coolant contamination. The "specs in the coolant are DARK black and brown, and the size of a pin head. Any assistance would be appreciated. I ASSUME the contaminants are rust and calcium, but I don't like to assume. Last info..Tstat is 180 and new,all hoses are new, coolant is same.
Thanks,
Derek.

The coolant was new when engine was placed in, and changed again when I changed the T-stat at 100miles (192-180 swap). The rad is full of deposits (white calcium,and some sludge bonded to the walls of the rad. The overflow (now JUST replaced with a new unit) was full of brown sludge, which I suspect is build up from the 200,000 miles put on the vehicle with original motor. The debris looks nothing like what coolant looks like when I add oil too it
(experiment I did in a glass of water)..there is no oily film..just black/brown little spots (lack of better word..to hard to get photo of) floating on the surface by the rad cap. I trust it is contaminants and dirt from the radiator itself, but just wondering what other indications of oil contamination would present. I will be replacing the radiator come fall, water pump is a polished Eddy and same milage as the motor.
Thanks guys for the time everyone. I know it's silly to spend all the $$ on the new motor and components and leave the high milage rad..but only so many projects per year...and that is (aluminum replacement) scheduled for next year.

I felt the same way...provided the cooling system upheld it's part of the deal it could stay..and considering 90 deg on sunday with humidity at 80% with the car never edging over 185-190 driving and low 180's at idle..I'll keep her intact for now. I FEEL the rad issue is dirt, and yes I am anal over the little things, but I do enjoy getting a little reassurance that all is as it should be. Thanks Tim, I'll keep you posted.