@article {robertson1961,
	title = {KNOOP HARDNESS NUMBERS FOR 127 OPAQUE MINERALS},
	journal = {Geological Society of America Bulletin},
	volume = {72},
	year = {1961},
	month = {Jan-01-1961},
	pages = {621},
	issn = {0016-7606},
	doi = {10.1130/0016-7606(1961)72[621:KHNFOM]2.0.CO;2},
	url = {https://pubs.geoscienceworld.org/gsabulletin/article/72/4/621-637/5310http://bulletin.geoscienceworld.org/cgi/doi/10.1130/0016-7606(1961)72\%5B621:KHNFOM\%5D2.0.CO;2},
	author = {ROBERTSON, FORBES}
}
