{"id":6307,"date":"2020-02-21T14:08:00","date_gmt":"2020-02-21T13:08:00","guid":{"rendered":"https:\/\/ssef.ch\/?p=6307"},"modified":"2022-11-02T15:30:58","modified_gmt":"2022-11-02T14:30:58","slug":"exceptional-colour-change-garnets-showing-the-usambara-effect","status":"publish","type":"post","link":"https:\/\/www.ssef.ch\/zh-hant\/exceptional-colour-change-garnets-showing-the-usambara-effect\/","title":{"rendered":"Exceptional Colour Change Garnets Showing the Usambara Effect"},"content":{"rendered":"\n<figure class=\"wp-block-image size-large\"><img decoding=\"async\" src=\"https:\/\/ssef.ch\/wp-content\/uploads\/2020\/12\/cc-1-1024x594.jpg\" alt=\"\" class=\"wp-image-6310\"\/><figcaption><em>Figure 1: Colour-change gemstones in daylight (always top left of the pairs) and incandescent light (always bottom right of the pairs). this picture has been combined from several pictures. the relative size of the stones is not maintained. the colours in daylight and incandescent light have been corrected (using Adobe Photoshop\u00ae) to best match the observed colours. <\/em><\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">  by Dr. M.S. Krzemnicki, first published in\u00a0<a href=\"https:\/\/ssef.ch\/wp-content\/uploads\/2017\/12\/SSEF-FACETTE2014.pdf\">Facette 21<\/a> (February 2014)  <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Colour change, also known as\u201calexandrite effect\u201d has been studied extensively since it was first described in 1831 for alexandrite (chromium-bearing variety of chrysoberyl) from Russia (von Pott, 1842). Since then, numerous minerals or gemstones with colour change have been documented (Figure 1), including corundum, spinel, zircon, fluorite, monazite, bastn\u00e4site, garnet, diaspore, kyanite, epidote and kornerupine, to name a few.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Traditionally, we speak of a colour change when the main hue of a mineral in daylight differs from that seen in incandescent light (LMHC 2010, Infosheet No. 9). With the advent of new light sources (fluorescent and LED lamps), and recently with the ban of incandescent tungsten bulbs, this definition may need further specification in future. The main factors for observation of colour change in a gemstone\/mineral are: a) two white light sources of distinctly different emission spectra (e.g. daylight versus incandescent light), b) a material that shows two transmission \u201cwindows\u201d in its absorption spectrum separated by an absorption band at approximately 570 nm, c) an observer whose brain interprets the incoming residual light energies into an according colour sensation (White et al., 1967; Nassau, 1983; De Valois &amp; Jacobs, 1984). Apart from colour change, there are further colour effects, which may considerably contribute to the colour perception of a mineral or gemstone, namely pleochroism (Liu et al. 1995) and the Usambara- effect (Halvorsen, 2006). Pleochroism describes the effect of different colours due to different selective absorption along the two or three vibrational directions within an anisotropic mineral (very distinct in alexandrite), whereas the Usambara effect is a colour change effect related to the path length of light transmitted in a stone (Halvorsen &amp; Jensen 1997).<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">In 2013, we received for testing a few exceptionally large\ncolour change garnets (50-100\nct) that showed\na distinct and attractive colour\nchange from brownish green in daylight to red in incandescent light\n(Figure 2). Based on the information of the suppliers of the stones, these came\nfrom southern Tanzania. Chemical and\nspectroscopic analyses confirmed that these garnets belong to the\npyrope-spessartine solid solution series with\nmarked concentrations of chromium and vanadium, well-known in literature to produce a colour change\nin garnet. We had analysed many similar garnets\nfrom East-Africa (Krzemnicki et al. 2001) in the past, but\nmostly in the\nrange between 1 &#8211; 5 ct and\nonly very rarely larger than 10 ct.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img fetchpriority=\"high\" decoding=\"async\" width=\"1024\" height=\"594\" src=\"https:\/\/ssef.ch\/wp-content\/uploads\/2020\/12\/cc-2-1024x594.jpg\" alt=\"\" class=\"wp-image-6313\" srcset=\"https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-2-1024x594.jpg 1024w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-2-300x174.jpg 300w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-2-768x445.jpg 768w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-2-700x406.jpg 700w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-2.jpg 1100w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><figcaption><em>Figure 2: two colour change garnets (100 and 57 ct) in daylight (left) and incandescent light (right) showing a distinct colour change (alexandrite effect). note the reddish brown colour along the girdle in daylight (left) in both samples, which in fact is a result of the usambara effect. this brownish red colour is due to the \u201cdoubling\u201d of the path length of light by internal reflections on certain pavilion facets .<\/em><\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Apart from their very outstanding size,\nthese new samples showed another very attractive colour change effect, the\nso-called Usambara effect. This specific\neffect, first noted by Manson\n&amp; Stockton (1984)\nas \u201ccolour-shift\u201d on colour-changing garnets was only\nlater fully described by Halvorsen &amp; Jensen\n(1997) on Cr-bearing tourmaline from the Usumbara mountain range, and thus\nnamed the Usambara effect.&nbsp; It describes\nthe property of a material to change colour in relation to the path length\nthat light travels\nthrough the material.\nIn the case of our garnets\nof equally greenish\ncolour in daylight,\nwe can observe a dramatic change of their transmission colour into red\nwhen held together and illuminated with a strong\nlight source from\nbehind (Figure 3). This specific \u201ccolour\nchange\u201d is not related to different white light\nsources (as is the alexandrite effect, i.e. classical colour\nchange), but is directly\nlinked to the different path lengths that light travels\nthrough the material when viewed alone or when put together (resulting\nin a \u201cdoubling\u201d of the path length).<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img decoding=\"async\" width=\"1024\" height=\"594\" src=\"https:\/\/ssef.ch\/wp-content\/uploads\/2020\/12\/cc-3-1024x594.jpg\" alt=\"\" class=\"wp-image-6316\" srcset=\"https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-3-1024x594.jpg 1024w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-3-300x174.jpg 300w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-3-768x445.jpg 768w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-3-700x406.jpg 700w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-3.jpg 1100w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><figcaption><em>Figure 3: Usambara effect (red transmission colour) shown with two superposed colour changing garnets (each brownish green in daylight). Garnets courtesy of anthony Brooke,<br> Gem Dreams ltd., Bangkok.  <\/em><\/figcaption><\/figure>\n\n\n\n<figure class=\"wp-block-image size-large\"><img decoding=\"async\" width=\"1024\" height=\"594\" src=\"https:\/\/ssef.ch\/wp-content\/uploads\/2020\/12\/cc-4-1024x594.jpg\" alt=\"\" class=\"wp-image-6319\" srcset=\"https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-4-1024x594.jpg 1024w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-4-300x174.jpg 300w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-4-768x445.jpg 768w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-4-700x406.jpg 700w, https:\/\/www.ssef.ch\/wp-content\/uploads\/2020\/12\/cc-4.jpg 1100w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><figcaption><em>Figure 4: alexandrite in daylight showing a greenish colour and brownish to red facet reflections along the girdle due to its strong pleochroism.  <\/em><\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Although garnets showing a classical colour change superposed by an Usambara effect have already been described (Manson &amp; Stockton 1984), it is to our knowledge the first time, that this effect is so easy to see &#8211; even without superposition of a second stone. Due to their exceptionally large size, these colour-change garnets display a brownish-green colour in daylight in the main part below the table (+\/- simple light transmission through the volume of the stone), but reddish facet reflections around the girdle as a result of increased (\u201cdoubled\u201d)  path lengths of light by internal reflections (Figure 2). This effect is not visible when illuminated by an incandescent light source, resulting in an overall reddish colour of the garnets due to their colour change (alexandrite effect).  <\/p>\n","protected":false},"excerpt":{"rendered":"<p>by Dr. M.S. Krzemnicki, first published in\u00a0Facette 21 ( [&hellip;]<\/p>\n","protected":false},"author":3,"featured_media":6310,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[626],"tags":[742,736],"library_category":[],"library_author":[],"class_list":["post-6307","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-research-blog","tag-gemmology","tag-othergemstones"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.9 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Exceptional Colour Change Garnets Showing the Usambara Effect - SSEF<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ssef.ch\/fr\/exceptional-colour-change-garnets-showing-the-usambara-effect\/\" \/>\n<meta property=\"og:locale\" content=\"zh_TW\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Exceptional Colour Change Garnets Showing the Usambara Effect - SSEF\" \/>\n<meta property=\"og:description\" content=\"by Dr. M.S. 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