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In market both natural and synthetic blue sapphires are available. Separation of natural blue sapphire from synthetic by naked eye is nearly impossible, even gemologist fail at certain level to separate them visually. The natural sapphires that we get in market are mostly treated (heat & diffusion). It is again difficult to identify these treatments with naked eyes.
First, never ever separate a blue sapphire from other blue gemstones only on the basis of shade, since blue sapphire comes in all shades of blue.
There are basically 5 gemstones that appear (replicate) as blue sapphire. These are Synthetic Cubic Zirconia (a.k.a. C.Z, American Diamond), Iolite, Tanzanite, Benitoite and Man-made blue glass.
Separation of Blue Sapphire from:
- Blue Synthetic Cubic Zirconia (C.Z.): Blue C.Z. is the most common substitute for Blue Sapphire. An experienced gemmologist or trader can easily separate Blue C.Z. from Blue Sapphire even by weighing these stones on hand; because if we take same volume of Blue Sapphire and Blue C.Z. then Blue C.Z. will be heavier as its specific gravity is more. Also, C.Z are often eye clean but sometimes have zirconium oxide “powder droplets” which can appear as rows of dots.
- Iolite: If Iolite is rotated from different direction it will show a shade of yellow which doesn’t appear in blue sapphire. So if your blue sapphire doesn’t show yellow shade then you can be certain that it is at least not Iolite. Note: Sometimes iron staining in blue sapphire can be the cause for yellow shade. So this is not conclusive evidence.
- Benitoite: Benitoite is a rare gemstone and it is less likely that you will encounter it. However, you can separate Benitoite with “doubling test”. When you look through facets of Benitoite towards culet, culet will appear as two. Even inclusions inside Benitoite will sometimes appear as two. This is a confirmatory test as there is only one natural blue stone with doubling effect.
- Tanzanite: Tanzanite shows three shades of blue when looked through dichroscope whereas blue sapphire will show only two shades of blue. You can also separate it with instrument called refractometer. Eye separation of tanzanite from blue sapphire is difficult. It requires certain instruments to get it separated.
- Man-made Blue Glass: Glass is a good simulant for all gemstones. All man-made glass has “gas bubble” inclusions which appears as round or oval or elongated. New people might find it difficult to identify & separate “gas bubbles” from “crystal fingerprints” but with few practice one can easily separate between “gas bubbles” and “crystal fingerprints”.
Blue sapphire (variety of corundum) is generally heavier than all these stones (Iolite, Tanzanite, Benitoite & Man-made Blue glass) except Synthetic Cubic Zirconia (C.Z.). That is, if the same volume of these gemstones are weighted blue sapphire will weight heavier.
Refractometer is an instrument which checks the R.I. (refractive index) of gemstones. It is a “life saver” for many people involved in gemstone trade. One can separate more than 50% of the gemstone solely on the basis of this small instrument. I personally suggest everyone involved in this field to buy this instrument.
A word of caution: If you are even 1% unsure whether your gemstone is genuine or not then it is highly recommended that you get it certified from a reputable/trusted gemological lab.
Updated On: December 14, 2012: Added Synthetic Cubic Zirconia as a substitute for Blue Sapphire and made minor correction to the post.