Colleagues, Matt Kohn, John Brady, and I have created a free open-source online optical mineralogy reference book. It is intended for students and others to aid them as they look at minerals in thin section. It is not an optical mineralogy text. You can find the book at https://optical.minpet.org/ The book includes descriptions of about 75 minerals. For each, we have emphasized what we see that lets us identify it. There are photos of 360 thin sections (PP/XP pairs). Most photos are ours, but we are very much indebted to those who let us borrow theirs - citations are in figure captions. The book also includes more than 50 video clips with voice descriptions. I hope you find what we created to be useful. Please let me know when you find the inevitable mistakes or omissions. Dexter Perkins Dept. Geology and Geological Engineering University of North Dakota Grand Forks ND 58202 (701)777-2991o; (701)757-0579h dexter.perkins@engr.und.edu