GRAYISH-BLUE and WHITISH to WHITE, semitransparent sapphires with silky lustre due to rutile microinclusions, called "geuda" in Sri Lanka, become transparent and acquire a beautiful, bright blue colour after calcination at temperatures of 1600☌ and higher.
Synthetic overgrowth can often be seen at girdle and culet: roiled appearance in transmitted light, light areas between cross-polarizing filters.īLUE – Heat-treated: dot-like partially dissolved “silk”, rounded melted crystals surrounded by discoid fractures, diffused colour banding, blue spots. Yellow-orangy once produced by irradiation: colour fading permanently if heated.ĭiffusion: colourless to yellow-orangy rim, blue haloes/spots (internal diffusion). YELLOW - Heat-treated: darken temporarily if heated (spoon and lighter 2 min or 15 min within 1 cm of 150W spot light). Colour zone following the shape of the gem (rim with distinct coloration from core): immersion in methylen iodide (or coconut/baby oil), diffused transmitted light, frosted complementary colour filters (blue for yellow-orangy). Colour concentration may be seen arround melted guest crystal. Most melted crystals are surrounded by discoid fractures often with dendritic pattern (partial recrystallization) they often show a cottony appearence. Resulting inclusions are distorted melted guest crystals such as white formless zircon (often with gas bubble). Very high heat treatment is required to diffuse the colouring agent into the gem.
Other colours may also result or be improved by this treatment (typically ruby, blue and green sapphires): standard tests are limited especially for clean light gems. In most cases, unlike natural, the rays do not show a 90° orientation to color zoning (crystal faces).ĭiffusion (beryllium): Most corundum with orangy hue, strong bi-coloration and saturated colours, typically yellow, orange, red-orangy, orangy-pink, pink-purple, hot pink, and padparadscha colour are beryllium treated. Fiber-optic light: shallow fuzzy whitish surface layer.
Star-diffusion(titanium): Surface-diffusion on a gem already cut into cabochon shape: strong star effect occuring at and just below the surface.
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