The latest high-tech equipment permits reliable results to be obtained even with microscopic samples.
Radiometric dating is self-checking, because the data (after certain preliminary calculations are made) are fitted to a straight line (an "isochron") by means of standard linear regression methods of statistics.
Early attempts at Rubidium-Strontium dating made use of the fact that the Sr-87/Sr-86 ratio in rubidium-poor samples was about 0.7.
Since these samples were poor in rubidium, it could be assumed that the primordial ratio of Sr-87 to Sr-86 was 0.7.
Thus, a precise measurement of the Sr ratio in a modern volcano can be used to determine age if recycled older crust is present.