This crystal doubles images, meaning that if you look through it that anything on the other side whether that be text, a person, or a photo of a volcano will appear to be doubled. But, why does this unusual optical property occur when utilizing a crystal of calcite? The answer is a fascinating phenomenon known as birefringence, which this video will explain through the expertise and analysis of a geologist. If you would like to support this channel, consider using one of the following links: (Patreon: http://patreon.com/geologyhub) (YouTube membership: https://www.youtube.com/channel/UCYeGh5VML5XPr5jYnzh3J6g/join) (Gemstone & Mineral Etsy store: http://prospectingarizona.etsy.com) (GeologyHub Merch Etsy store: http://geologyhub.etsy.com) Google Earth imagery used in this video: ©Google & Data Providers This video is protected under "fair use". If you see an image and/or video which is your own in this video, and/or think my discussion of a scientific paper (and/or discussion/mentioning of the data/information within a scientific paper) does not fall under the fair use doctrine, and wish for it to be censored or removed, contact me by email at [email protected] and I will make the necessary changes. Various licenses used in sections of this video (not the entire video, this video as a whole does not completely fall under one of these licenses) and/or in this video's thumbnail image (and this list does not include every license used in this video and/or thumbnail image): CC BY 2.0: https://creativecommons.org/licenses/by/2.0/legalcode CC BY SA 3.0: https://creativecommons.org/licenses/by-sa/3.0/legalcode Sources/Citations: [1] Mindat, https://www.mindat.org/ Note. All mineral data, including birefringence and crystal structures, and the number of isometric minerals were cited from Mindat.org [2] Mindat, "Pharmacosiderite", https://www.mindat.org/min-3185.html [3] Mindat, "Rutile", https://www.mindat.org/min-3486.html [4] Mindat, "Geikielite", https://www.mindat.org/min-1670.html [5] Mindat, "Cinnabar", https://www.mindat.org/min-1052.html [6] Mindat, "Carpathite", https://www.mindat.org/min-2162.html [7] Mindat, "Wattersite", https://www.mindat.org/min-4248.html [8] Mindat, "Calcite", https://www.mindat.org/min-3185.html [9] Mindat, "Zircon", https://www.mindat.org/min-4421.html [10] Mills, S. & Kampf, Anthony & Williams, P. & Leverett, Peter & Poirier, Glenn & Raudsepp, M. & Francis, C.. (2010). Hydroniumpharmacosiderite, a new member of the pharmacosiderite supergroup from Cornwall, UK: Structure and description. Mineralogical Magazine. 74. 10.1180/minmag.2010.074.5.863. 0:00 A Cool Phenomenon 0:29 Light Waves 0:42 Birefringence 2:38 High Birefringence 3:19 Pharmacosiderite 3:43 Other Optical Properties