Okay, more close. I will bring you far away from labs and queen stabilizer. We go until Axum in Ethiopia. And the talk is about opal, which is a precious stone, a gem, natural gem.

But it can be said also it's a colloidal crystal or photonic crystal and or periodic grating. So it's why we name with Pascal this pseudo crystal beginning.

And of course, it's dedicated to Dr. Steve Benton. And most of you knew the piece, but everyone could see the piece yesterday evening as a castle, which was a famous piece in 1977. Okay, so the trip is you take a plane,

you go to Addis Ababa, done about Abyssinia, a typical view of a street in Addis Ababa. Now this is the bus station towards north, the street we are going to take.

So Addis Ababa, the capital is here.

And you know certainly that Ethiopia is a place of the Rift Valley, the famous Rift Valley. And there are three rifts, the Red Sea, the Gulf of Aden, and the Ethiopian Rift. But the Ethiopian Rift didn't open completely and didn't give a sea. But it's a big depression where you can find big lakes here.

And each side you have high mountains going until 4,600 meters. This means that even close to Ecuador, you are in an alpine climate, it's called. It's not the case here where you have 100 meters beneath

the level of the sea. And we take this street of the north and we go here, M. It's a village named Meseso, strange, Meseso. It's 3,000 meters high in the mountain. I travel with my two friends, Gebo and Yasu.

The big one is Yasu Bekele, my friend and partner. And he has a list of near 200 kilometers square when I can prospect and take the opal. So this is a rival in Meseso. The video is not working, but it's OK. This village, among three others, like Chela Dinge,

is a village in the core of the opal deposit. And so here we are very high, 3,200 meters. And you can see the cliffs going down to the reef valley. And in places like this here, we can find the opal.

So the landscape is really fantastic. And people lived there like 2,000 years ago. All this green field of teff, a cereal, which is surviving in the high climate in altitude.

We had only one CD in the car. It was Bob Marley. But I remember you that Rasta, Rastafari,

is coming because Aile Selassie, which means the power of the Trinity, first name when he was young. He was governor of O'Hara. And governor was Ras, and he's named Tafari. So it was given Rastafari. So the opal are formed mostly by the shepherd and the people

living in the mountains. There is no mind, no machine. For the moment, it's very new. So about color, we began to see color in the stones. But Abyssinia is full of very strange things. This is an endemic bird. It's a superb starling, and there are different spices.

This one is big, near like a crow. In shadow, he's black, but when he goes in the sun, he has wonderful blue color. And this color are, of course, interferential colors, physical color. It's not a pigment, and it's just beautiful.

But this blue is found in the opal we found there also. All these opal are found in nodules, and the blue is in the opal. So we will see like a rainbow, but it's not really a rainbow. Each color is linked to a special size of sphere.

In an opal, sphere are packing and ranging perfectly. And you need maybe two micrometer or three micrometer of spheres to having bright colors. And the Bragg law is very well to explain this,

like in hologram. And when roughly when you are a sphere giving a diameter of fifth of a micrometer, you have the blue. Under is diffusing. It's like in holography, OK? And by the way, when we describe an opal, she is fuzzy. She has a very pure color.

She is noisy. It's like hologram, OK? And at the end, after a third of a micron, you go to the infrared. But infrared opal can be very bright, but you don't see them. You need a detector.

We go to the structure. These photos were taken in France in labs in Lyon. And in a knot, Professor Fritsch and Professor Gautier. And I supply them for now, oh, six years ago with samples. And you can see the structure, which is very close to what fringe we can see in a hologram.

This sphere range perfectly. We go down. You see, now we have one micrometer here. We still go down, more or less. And this was very interesting. The holes, because we take over to do very thin slices

for the MAB, the silica was taken over by the fluoridic acid. It leaves holes. And it's like the beep. You can see the periodicity. But this sphere in opal from Ethiopia are not simple sphere. They are done by little sphere, as you can see,

like a raspberry, you know. It's, we say, a leposphere of thromboedal. And by the polarization photography, we can see also the structure. If you notice the lines, the grain, the structure like this, like the mouse, it can be called a grain, like in crystallography.

In fact, opal reproduce everything you can find in crystal with atoms. But it's another level. It's not nanometer, but micrometer. You see very well the grains. And you see all these lines perfectly parallel. You see the steps.

You can see the steps and the little sphere giving the diffraction. OK, it's linked. So this nodule can be very big. And you can find all kind of opal inside. White opal, crystal opal, black, I said, brown.

But the most interesting is this kind of opal for us, because it has a very strange microstructure, like a honeycomb. And in fact, we must see that, I shall do after. This honeycomb structure has a cut of the opal by the side,

but correspond to tubes. And the tubes opal form perfectly. By the way, and this is interesting, this piece I take from Paris coming here, because we did it for a French jeweler, Natalie Castro. Right now, in France, there are 20 jewelers doing creative piece upon opal.

But Natalie choose to mix holography and opal. So she tell me, you can speak a little, but not too much. OK, because she has two creations in the way. So imagine a ring, big gold, with nice flower

done in gold with diamond, and a crown with emerald. And in the crown, you have this opal, then a hologram done by Pascal, which is a bee. And a quartz on top to finish. So mixed media.

And if we look close, and this is interesting, this opal have very strange properties. I spoke about the tubes. They can be very little. And when you look to the opal, you have like stars. You see, when the tubes are little bigger, the grain, you can clearly see the surface, you know, it's big.

But some pieces, some opal, the whole opal is one mono crystal. This means that through all the opal, centimetric, the sphere are perfectly arranged. And this gives a very strong properties. And this piece on the top is polished and prepared for microscopy. But this piece is rough.

And you can see that only by tilting the stone until a halogen lamp, the image of the point of the source is traveling from here to the middle to here, just by tilting. And if I polish the piece, this is very interesting. The polish is giving a normal mirror image.

But you can see here a very nice image given by the opal, like a hoe. It's like a reflection hologram, but with diopters, like a lens also. And if I go under a neon, which is a light with not a continuous spectral,

but with some ray, you can see you have the green of the neon, the orange, but this is black because it's missing. It's not the case under the sky where or here with the lamp, the spectrum is continuous. So this opal can be used like a spectral analyzer. And in this case, I just hold my hand under the sky,

and it's an image of my hand. Some of you did the experience the other day during lunch. So this is a big image of this stone, naturally giving an image of the point source.

I see you see well here. No doubt we have the image of the facet of the halogen lamp. But to the light in the mountain, we go to see the place, now a little geology. When we are at this level, we are above four kilometers of lava coming from the activity of the Rift Valley.

And in some stage, some lava were very rich in silica. And this silica brought the silicium to form the opal, which is a CO2 with a combination of molecular linked water, okay? And some lavas has holes, like the Pierponts, I don't know the word in English.

And afterwards, opal dissolving silica began to form, water began to form with silica, opal in the holes. And the weathering is throwing away the lava, and you have the nodule with the opal. And this is the mine. So I said, no machine, nothing. Among the tough fields,

you can find opal oil cropping like here. So this was given good opal. And you can see, perhaps here, but better after, the rocks are full of holes. And the holes, my little friends in the mountain, you see here in a very weathered ground,

the nodules, like shells, you know, like clamshells. You see here? And here, opal oil crops, one can take this piece here and throw on the ground. If it's a good opal, he will see color. And I often said to you the other day, I'm sure that in the past 4,000 years ago,

a shepherd could take such a nodule, break it, and see the colors, and even notice image of clouds in color in the stone. But it was only a few years ago that it was bring to our knowledge.

In fact, there are some historical points. Now, a little history, and strangely, it's linked to Great Britain. You all know Dr. Leakey, who was a very famous anthropologist searching for early human life in Africa. And he found in a cave in Kenya

what he called the first opal artifact. And he wrote this in the Mineral Yearbook in 1939 in New York. And he said this opal, he dates the cave and the activity of the human with the stone from 8,000 years from now. And he said the opal was coming from Ethiopia.

Then it said in some books now of gemology or so in Tucson, you can read about Ethiopian opal because it's coming out very seriously. But it still said the first appearance on the market was 400 grams on the gems market in Nairobi in 1993.

But recently in Paris, I give a conference in the center, and John Sowell, which is a famous discoverer of Ribey Mine in Tanzania in Africa, told me a man from Ethiopia came to him in 72 and brought through this kind of stone. And he said, but it was like what you're showing now.

And I gave the stone to the British Museum, so we have stones from 72 in London. So you see here the nodule is a rock, but sometimes opal is not feeling completely the nodule.

And this gives this very strange structure in the surface. And this now is going to interest us because it's the beginning of the structure. It's like when you do a soup Sunday and you put it in the fridge for Monday. The fat is freezing on the top. Did you notice this? And it's giving some valet mountains. This is the same thing, but not in the Sunday night,

but over a thousand a year perhaps. So the colors are still amazing. This is a contra-loose piece, meaning that when you take the stone light by the top, you have no color. And when you put in for the light going

to be transmitted to the stone, the color appearing. We don't really know why. Perhaps it's because the sphere are big and it's infrared. So we have diffraction by transmission, but not by refraction. But we didn't, we have no proof. Still work to do. The case of this stone, still contra-loose.

Some very strange patterns sometimes, like damascuan iron. Fire opal, sometimes. So opal is transparent with trace of iron and it's yellowish or orange as are very valuable.

It is named fire opal and not to be misunderstood. So fire of the opal is not fire opal. This is a fire opal and with green fire, with green play of fire in it.

You see all kind of colors. This one, for the painters. When I saw it, I say, wow, it's a Miro. And it's untouched, it's natural, okay? So the module was not completely full of opal. And we discover, I did not find all these pieces.

Of course, the people of the mountain, I go there and they bring me the stone and my friend. So but when I see this, wow, it's a painting. But when you reverse the stone. So the stone was with layer, okay? This was on the top and it was a kind of separation.

And when I turn it, I had this. And these are like, I am sure of what I'm saying. As I said, still big work to investigate. But these are like vertex, vertical vertex, presenting as a formation of the tube, which are going to have these optical properties.

But what's fascinating, this structure can perhaps, I don't know, take 10,000 years to be done. And it's revealing the forces doing the opal. Because when sphere are going to pack, the force is linking them, okay? But this is happening in a perfect sphere, the nodule.

And this bring also some forces. And the result is this. So this kind of opal can help us to understand. It's still like this, I think I have it here. And if we slice it for the microscopic electronic, electronic microscopic, we can find something of course.

Of course, it's not sure here. So again, I show the structure. And also sometimes when the opal is not completely full, we have a kind of, when it's drying, a kind of depression, okay? How much time do I have, five minutes? Okay, so I go.

So you see the depression in the opal. This one also. And here, the tube is vertically thin. And here, if you do a cut. But with the depression, the pack of sphere, which was a vertical plane, can bend. And you will have perhaps a Fourier, a micro-Fourier structure, a Fourier, a Fresnel structure.

Okay, and this giving the optical properties. Here, you can see in a crystal opal transparent, the tube seen by the top, and here by the side. And whether the spheres are packed like this, or like this, you will have an opal. If you cut a piece in a tube plane,

you can have a real image. If it's plane, you can have a perfect plane image. And if it's in this size, you can have a real image. And really, we can see the light floating above the opal, like this. This one is now in New York. A jeweler purchased it because it was extraordinary. You see these little points

are floating one centimeter above the stone. And you have dozens of points giving like a field of color above the stone. Real image. Image of a halogen lamp here. Okay, this I go quick, but this is like when you bleach a hologram in a very bad manner, and you have two lengths of fringe between,

so it's diffracting like green and red at the same time. Or happening in dichromatin gelatin, too. And so, you have fantastic patterns. I go quick.

See the tubes, but in a white opal, you can see. When you rotate the stone, it's like stars. This one giving a very rare yellow. Yellow is very difficult to see in an opal. Like in holography, a good yellow. You know, it's linked. Without some reason.

When Tom show us one of the slide before, I say, wow. You remember? The hologram with the patterns, you see?

Even in rough pieces, you can see the optical effect. It's a piece before cutting and polishing after. And now, to finish, we go back to this piece. You can see the cells here are very well done. I was lucky. I saw myself this piece with a slab saw.

It's a very thin diamond saw. I opened, I saw very pure color. So I said to Jean-Pierre, gear in Strasbourg, my cutter. Polish, polish, and I go to Hini. I introduce the stone to the Hini laser, a little Hini laser. And I concentrate on this grain, which is this one here. Excuse me.

This one, okay. This is a result. And so, coming from the right top is a beam, so reflection on the polish of the stone. But the beam coming towards us is diffracted by the opal. And it's diverging less than two degrees.

So this is amazing because it's acting like a very, very good optical crystal, okay? So it's time to say hello. And for the discussion,

we can go to the very famous tree in Aksum, but we can also discuss with a glass or something. Okay, thank you. Any questions, Francois?

No? Okay, that was great. Thank you.