[chris (OP)]

Anton has written to say:

Hi I am from Sri Lanka  I would interesting to make device for find gem stone underground from 10 crt onward like blue sapphire yeallow. Green white also the ruby some other veritys could you tell me what's is frequency this gem stone Cary underground size and the depths

[Bored chemist]

If I knew a way to do this
I would be very rich and
I wouldn't tell anyone.

However I can explain why I think it's more or less impossible.
I would like to be able to find sapphires.
Sapphire is a variety of aluminium oxide.
In the aluminium oxide crystal., all the aluminium ions are surrounded by oxide ions.
The trouble is that in feldspar- one of the commonest minerals on earth, the same structure- aluminium ions with oxygen ions is also resent.
The same is true of clay, and lots of  other minerals.
So, from most points of view sapphire looks a lot like clay.The only real difference is the one that's "obvious" to us- it lets light through it.
Well, the only way to check that is to have a light source on one side, and a detector on the other.
And you can't do that while it is buried in mud- because the mud blocks the light.

[Kryptid]

Some kind of device based on sonar might help: sending sound waves into the ground and looking at the reflection to determine the various densities of the underlying materials. You'd just have to hope that the gemstone you are looking for doesn't have a density equal to that of some other, less valuable mineral.

[Bored chemist]

Some kind of device based on sonar might help: sending sound waves into the ground and looking at the reflection to determine the various densities of the underlying materials. You'd just have to hope that the gemstone you are looking for doesn't have a density equal to that of some other, less valuable mineral.

You also have to hope that there's not something like water in the way which would attenuate the ultrasound, nor sand which would scatter it.

[jeffreyH]

Quantum radar?

[Kryptid]

You also have to hope that there's not something like water in the way which would attenuate the ultrasound, nor sand which would scatter it.

I wonder if some of those problems could be mitigated by using multiple shockwave generators placed at large distances from one-another and each sending different wavelengths of sound into the ground? Some wavelengths would probably be affected by obstructions differently than others, which might lead to a detectable difference in the reflected signal.

[evan_au]

I understand that diamond miners use X-Ray fluorescence to show up diamonds.

The X-Rays would penetrate the ground to some extent (if you have a powerful-enough X-Ray source).
But the visible-light fluorescence does not pass through the ground very well
... unless it is through sand - and I have seen photos of people prospecting for diamonds in the desert; a row of destitute people, lying on their stomachs in the sand, and examining every grain of sand in front of their faces. I am sure they were paid a pittance!

That's why diamond miners dig up the ore and crush it before doing their X-Ray fluorescence trick.
See: https://en.wikipedia.org/wiki/Diamond#Mining