Brigham Young University:

Scientists associated with BYU have created some notable inventions. Philo T. Farnsworth, inventor of the electronic television, received his education at BYU, and later returned to do fusion research, receiving an honorary degree from the university. Harvey Fletcher, also an alumnus of BYU, inventor of stereophonic sound, went on to carry out the now famous oil-drop experiment with Robert Millikan, and was later Founding Dean of the BYU College of Engineering. H. Tracy Hall, inventor of the man-made diamond, left General Electric in 1955 and became a full professor of chemistry and Director of Research at BYU. While there, he invented a new type of diamond press, the tetrahedral press. In student achievements, BYU Ad Lab teams won both the 2007 and 2008 L'Oréal National Brandstorm Competition, and students developed the Magnetic Lasso algorithm found in Adobe Photoshop. In prestigious scholarships, BYU has produced 10 Rhodes Scholars, four Gates Scholars in the last six years, and in the last decade has claimed 41 Fulbright scholars and 3 Jack Kent Cooke scholars.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What did Tracy H. Hall invent?
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