Geological Information

Geological of Corundum Family

• Family includes: Sapphire, Ruby
• Ruby has fluorescence! Sapphire can show star patterns!

General Identification

• Type: Mineral
• Class: Oxides
• Member of Group: Hematite
• Mohs Hardness: 9
• Formula: Aluminium oxide, Al₂O₃
• Crystal System: Trigonal
• Crystal Habit: Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular
• Lustre / Diaphaneity: Adamantine to vitreous, Transparent, translucent to opaque
• Colour: Colorless, gray, brown; pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown

Ruby

• Gem stone, the red colour is caused mainly by the presence of the element chromium.
• Ruby fluoresces with UV light. Regular corundum does not.
• Has a variety which shows a three-point or six-point asterism or "star" under certain light conditions amd known as "Star Ruby".
• Red corundum is often sold as ruby.
  • It may even be coloured and is often brown corundum.
• Red spinels, red garnets, and colored glass have been falsely claimed to be rubies.
• Quality is improved through heating, improving transparency by dissolving rutile inclusions, healing of fractures (cracks) or even completely filling them.
• More geological information.
• Colours: Red. Pink colour is debated as to whether it will be classified as ruby or sapphire.

Sapphire

• Colour determined by trace amounts of elements such as iron, titanium, chromium, copper, or magnesium.
• Mainly used for jewellery, abrasives, infrared optical components, high-durability windows, wrist-watch crystals and movement bearings, and very thin electronic wafers for very special-purpose solid-state electronics.
• Has a variety which shows a "star" under certain light conditions amd known as "Star Sapphire".
• A rare variety is known as color-change sapphire, exhibits different colors in different light.
• 95% of all the world's sapphires were being heated to enhance their natural color.
• Can be confused with Iolite which is called "Water Sapphire" to add to confusion. But iolite shows as a grey tone when looked from the side.
• More geological information.
• Colours: Any color except red. Best known for blue.

Emery

• Black granular corundum intimately mixed with magnetite, hematite, or hercynite

Geuda

• Form of the mineral corundum, or sapphire, found primarily in Sri Lanka. Around 70%-80% of gems mined in Sri Lanka belong to geuda varieties.
• Semitransparent and milky appearance due to rutile inclusions, these stones have little value as gemstones in their natural state.
• Once heated they rival other natural sapphires for look.

Special Notes

• It is commonly used as an abrasive on everything from sandpaper to large tools.
• High quality versions are part of the precious gem category. Cost of natural crystals varies depending on their colour, clarity, size, cut, quality and geographic origin.
• Nowadays most sapphires and rubies now sold in jewellery stores are synthetic
  • same composition but lacking healing qualities of natural grown pieces.
  • jewelers prefer synthetic as colour is consistent and cutting and setting has little to no risk of damage as there are no imperfections.
• "Ruby in XXX" where xxx is fuchsite, kyanite and/or zoisite is with real ruby.
  • All components are visible but which name used depends on what the mines or wholesalers felt was the more sellable name.