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LuxeSci Show Notes: S1E15: Sapphires

Hello again.  Welcome back to LuxeSci - a podcast to re-ignite your wonder by exploring the science behind luxury items.  I’m Dr Lex, PhD, infectious disease expert and jewelry lover.  I’m joined by my co-host Dr. Dimos, also a PhD, electrical engineer and materials science enthusiast. This episode we’re delving into the science behind one my favorite gems, the sapphire.  

Dimos - what is something you wondered about sapphires before this episode

Me - I wondered about how they can come in so many different colors.

Speaking of colors, I love both the deep blue tones that are usually associated with sapphires and the variety of colors they take on.  I also have a special memory connected to sapphires.  Back when I was a baby masters student, I spent a little over a month in Cambodia volunteering in the microbiology labs of hospitals there.  Cambodia is among the countries that gemologists and others source gemstones from.  One of the nurses at the hospital told me that she knew of a jewelry dealer in the local market that had beautiful pieces at a price that a grad student might be able to afford (again, closer to the source, local craftsman, etc).  We walked through the whole market, or so it seemed, until we arrived at a small stall way in the back with an ancient-looking old woman as the proprietor.  I spotted a gorgeous sapphire ring, white gold, with three perfectly matched oval sapphires in a bright blue.  Now, here in the US, rings are stamped with 14K gold or 18K gold.  This ring didn’t have a stamp so there was no way in the moment to verify that it was indeed made of white gold and sapphire, but I trusted both the nurse and the vendor and it looked real to me.  We decided on a price that was fair for both and that I felt comfortable spending, even if it was really just a good imitation.  When I got home was the first time i wore the ring for any extended period of time.  As we found out in our silver episode, I’m allergic to pretty much any metal that isn’t gold.  I wore the ring for a few days and no reaction!  At least I know that the band was white gold.  I’ve got my own internal quality check!  The ring is indeed real and represents a milestone for me, not just as a memento of an incredible trip but also as the first time I bought myself some nice jewelry.

Dimos - what’s your story about sapphires

My experience with Sapphire is with watches. I got a nice skeleton citizen watch for my high-school graduation. Of course i dropped it on the sidewalk as i was fumbling to put it on. It hit the ground on the curb right on the face of the watch. As it clattered to rest I picked it up and noticed that it did not have a single scratch. I was amazed and relieved, it was the inherent hardness of the sapphire crystal.


  • Sapphire belongs to the family of the mineral corundum

  • Corundum is a crystalline form of aluminum oxide

  • The other member of the family is ruby

  • Corundum is usually clear but takes on colors depending on transition metal impurities

  • Sapphires are usually associated with blue stones but can come in a range of colors

  • The blue color comes from intervalence charge transfer.  This is from the transition metals present in blue sapphire (iron and titanium).  An electron is transferred from the iron or titanium ions and that causes a change in the valence state of both

  • Valence state - defined by how many hydrogen atoms that element combines with and is the affinity or combining state of an atom (essentially its capacity to bond with other atoms)

  • The change in valence state requires energy and in this case electromagnetic energy is absorbed with the wavelength of that absorbed energy being yellow light.  As we know when there is a certain color light absorbed, we see the opposite color - thus the blue appearance of these sapphires

  • More iron = darker blue

  • Pink sapphires and rubies are caused by chromium impurities

  • Purple sapphires have traces amounts of chromium and iron plus titanium (remember your kindergarten lesson, blue and red makes purple)

  • Parti sapphires contain two or more colors in the same stone.  Interestingly, particolored sapphires cannot be created synthetically

  • Padparadscha - pink-orange sapphire originally found in Sri Lanka and seem to be created by a diffusion of beryllium

  • Star sapphire - exhibits asterism

  • Intersection needle-like inclusions that follow the underlying crystal structure lead to the appearance of a six-ray star pattern when viewed from above.  

  • The inclusion is often rutile (a mineral primarily made of titanium dioxide)

  • Sapphires are mined from alluvial deposits

  • Loose clay,, silt, sand and gravel deposited by running water in a stream bed or similar setting

  • Mined all over the world, including here in the US

  • Mostly from Montana and a few have been found in Franklin, NC

  • The word sapphire comes from French saphir and Latin sapphirus and sappirus in Greek

  • The Greek term was likely used to refer to Lapis lazuli and not sapphires

  • During the Middle Ages, European lapidaries used the Latin term sapphirus to refer to the blue colored corundum crystal

  • Lapidary - practitioner of shaping stone, mineral and gemstones into decorative items

  • Sapphire is birth stone for September and the traditional gift for a 45th wedding anniversary

  • Italian superstition holds that amulets of sapphire guard against eye problems and melancholy 

  • Science

  • First, we have a little throw back to a previous episode on diamonds where we talked about chemical vapor deposition (where material is vaporized, put in a vacuum container and then deposited in thin films)

  • I love a throw back fact.  It reminds me that science is all very much connected and that once you learn the terminology, it unlocks a whole world of discoveries

  • OK - back after that philosophical interlude

  • It turns out that you can make graphene film electrical biosensors on a sapphire substrate

  • Joe DJ, et al published this method in an article in 2016 in the Journal of Nanoscience and Nanotechnology

  • Just so we’re all on the same page, graphene is a 2-D single or few sheets of hybridized carbon atoms

  • The researchers grew the graphene directly on a sapphire substrate (substrate = surface or material on or from which an organisms lives, grows, etc and also a substance that an enzyme acts on)

  • They used a double sided polished sapphire wafer (0.43 mm thick)

  • The gases they used were methane to give a higher quality film growth and hydrogen since it controls the rate of growth of the graphene

  • These sheets of graphene on sapphire were used in experiments to detect cancer cells

  • The graphene/sapphire sheets were treated and coated with an antibody that would detect EpCAM (epithelial cell adhesion molecule)

  • When the cancer cells are present, they bind to the antibody and that changes the conductance of the sensor, which can be measured

  • The use of low electric fields didn’t damage the cells or degrade the antibodies and was able to detect changes in resistance as large as 20% with a small number of capture cells

  • The researchers used sapphire as the substrate (as opposed to other podcast topics, like quartz) because it is suitable for high temperature graphene growth.  It’s stable up to 2000C

  • Recap = graphene is grown on a polished sapphire sheet and treated to be a sensor and with antibodies to a specific cancer cell.  When the cells bind to that antibody, it changes the resistance of the graphene sheet and that can be detected and measured, thus quantifying the cancer cells present

  • The researchers are hoping that this type of technology could be used for real-time cancer cell detection

  • Another type of biosensor developed using sapphire is a sapphire sphere photonic sensor

  • Now, i may need to go back to school to get a PhD in sensors and optics just to decipher this article but i’ll do my best to summarize

  • Optical biosensors have some advantages over other detection devices (think PCR for detecting DNA, etc)

  • The can do remote sensing

  • Extremely sensitive

  • Nondestructive to the sample

  • The authors of this paper (Mohammed Murib, et al J of Biomedical Optics 2014) used a photonic microcavity (reflecting faces on the two sides of a spacer layer)

  • The target molecules are amplified due to the recirculation of the light in the microcavity

  • They also change the properties of the microcavity as to be detectable (via an electromagnetic wave)

  • The authors created a sphere of synthetic sapphire and inserted a probe DNA (meaning they knew the DNA sequence already)

  • To detect the DNA the team used a diode laser to excite the electromagnetic wave produced by the DNA inside the microcavity and the scattered light was detected by a photodiode via a optical microscope and the data sent to an oscilloscope for data acquisition.  (this is sort of akin to an X-ray where scattered light is detected and imaged and that image is used as a source of information about the system)

  • The authors concluded that this system could be used to study the physical properties of DNA and protein molecules

  • However, i think it would be cost prohibitive to be a real sensor to detect DNA or RNA

  • More recently, Xia P et al published in Biosensors and Bioelectronics their experiment on sapphire-supported nanopores for high speed, low-noise DNA sensing so perhaps the field is moving forward 

  • Finally - case of science self-correcting.  

  • Single crystal sapphire aluminum oxide ceramic dental implants were developed in 1972 as an alternative to the metal ones

  • While ceramic is biocompatible and has good compressive strength, it is hard and brittle

  • The thought was that adding the sapphire aluminum oxide would offset some of these disadvantages

  • Unfortunately, the new sapphire implants had a range of problems including, fractures, infection, pain, bone loss 

  • One follow up study of patients with porous sapphire dental implants found that 3 of 65 implants remained in place 20 years after they were installed

  • They are currently not used anymore in dental practices


Aluminum Oxide: is the core material used to create aluminum metal, also known as Bauxite

Alumina is commonly added to glass for strength and durability.

It is used as a filler and even in sunscreen

Inexpensive substitute for industrial diamond having a hardness of just 1 below diamond.

Applications: Thermometers and Lasers 

Lasers: Ti: Al2O3 is a titanium doped sapphire crystal laser. one of the first lasers developed with this material was in 1982 @ MIT Lincoln Labs.

Normally temperatures are measured using thermocouples, the next level of sensing is with commonly used fused silica optical fiber sensors. These however exhibit severe limitations at ultrahigh temperatures due to significantly degraded optical and mechanical properties at temperatures 1000 °C. Sapphire fiber sensors with a melting point of 2040C are much more capable of measuring systems such as the burners in powerplants to help improve combustion efficiency. Ultra high temperature measurement using Sapphire Fiber Bragg Gradient:

Also pressure measurement @ both 1000°C and 1000 psi.

White-light interferometry is used to determine temperature.

  • Glossary (going to try a little different tack this time and give my non-scientific version of a definition)

  • Valence state - how much an atom can bond with another atom (usually hydrogen)

  • Alluvial deposits - gunk washed down rivers, etc that can contain sapphires

  • Lapidary - someone who shapes gemstones

  • Graphene - thin sheet/sheets of carbon

  • Substrate - something to grow stuff on or something an enzyme acts on

  • Fun facts

  • What makes a purple sapphire (chromium, iron and titanium)

  • Why is sapphire good for making graphene sheet biosensors (high temperature tolerance)

  • What are sapphires made of (aluminum oxide)

I hope you’ve enjoyed exploring sapphires with me and Dimos and I hope you remember a little fact from this episode the next time at your next party, maybe if someone just had a dental implant.  Thanks for listening to this episode of LuxeSci.  A very special thank you to my audio engineer and co-host, Dimos.  Our theme music is Harlequin Moon by Burdy. We’re on Twitter and Instagram at luxescipod and our website is  While we always love for you to follow us, subscribe to the podcast and review us, this week we’re asking that if you’re moved by what has happened recently here in the US, please call your Senators and representatives and voice your support for gun control legislation.  If you want to know more about how to do that, you can visit  Our hearts are with all those who are healing from the unspeakable tragedy of the last few weeks.



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