In a diamond-anvil high-pressure cell having a tubular piston and a cylinder in which the piston is slidable to effect compression of a pair of opposed diamonds located between said piston and said cylinder, said piston including a central bore opening on one end, an adjustable X-ray collimation system comprising a tubular insert engageable in said bore of said piston, said insert including a central bore and having first and second ends, with said first end of said insert being closest to the said opposed diamonds and said second end of said insert extending out of the open end of said piston, a collimator insertable in said bore of said tubular insert, said collimator having a central bore and having first and second ends corresponding respectively with said first and second ends of said insert, elastomeric pivot means mounted in said bore of said insert at said first end of said insert for flexibly retaining said first end of said collimator while allowing said collimator to pivot within said pivot means, and adjustable locking means located at said second end of said insert for adjusting and securing said second end of said collimator so as to be in alignment with said opposed diamonds.