# Can I build a circuit to measure the resistance of common gemstones?

I would like to build a small, portable device that (as part of its operation) measures the resistance of common gemstones.

I don't care about accuracy - I don't want to try and identify what sort of stone it is, I would just like to try to roughly identify individual stones - to behave as a sort of pretend "recharge" mechanic (put a little tourmaline in, get 5 charges, put a ruby in get 10, but can't put the original tourmaline back in again).

Is this feasible in a small, mostly bare circuit board (don't want to put it in a case if I can help it)? I did see some suggestion that I could use very high voltages, but I'm not experienced with using high voltages and I don't want to end up accidentally killing myself.

• The resistance depends on the physical dimensions. Also I'd expect most of the gemstones to be a very bad conductors as they have a pretty rigid crystal structure. Commented Aug 30, 2022 at 19:03
• The conductance of moisture on the gemstone surface will likely outweigh the gemstone's own tiny conductance. Would an optical approach suit your application? Commented Aug 30, 2022 at 19:22
• Multimeter probably only measures up to 20MΩ. Gemstone would probably be in the GΩ range. Commented Aug 30, 2022 at 19:29
• Resistance depends on probe contact surface area, pressure, etc. Metal-metal contacts are quite low resistance and repeatable, but basically, if you stick a probe into a high resistance material, what you'll measure is contact pressure... Commented Aug 30, 2022 at 19:34
• Conceivable you could measure CAPACITANCE. Maybe. Possibly. Capacitance meters are cheap, give it a shot Commented Aug 30, 2022 at 20:54

There are no repeatable electrical characteristics that could reliably detect any gemstone, considering R,C, and unlikely but possible resonant frequencies. Size, shape, electrode contact area and surface contaminants will dominate any measurement with great uncertainty.

Pure Quartz and Diamond are pure insulators with resistance like glass. Not very revealing. Surface contamination and electrode contact area will not give repeatable results for many measurements around an irregular surface. Thus resistance is not a reliable indicator with moisture and dust adding to conductivity error.

Gemstones are similar except have slightly more internal conductance from inclusions of trace metals and other materials that create colour to sunlight.

I would think gemstones could be analyzed better under different light sources such as UV, And RGB laser colours differences between synthetics and real gems if one has a library of all the trace metals that affect light in clear quartz or Aluminum oxide .

Most minerals are very good insulators, galena, graphite, and gold being the three exceptions that I am aware of.

If this is for a game and you only want to identify individual stone game pieces, drill them and place implantable RFID tags inside them. (there seem to be prices down to \$1 for animal tags in small quantities)

If this is for some sort of geology quest game using found gemstones you're probably going to need a multi-spectral camera and some non-trivial software.

• Thanks. I had a silly idea to create a modern take on divination/tarot - I want to create a little playing card size PCB with a bunch of different LEDs mounted in Important Configurations, and thought it might be cool to Tune or Charge it with various crystals to make Readings. I might drop that last part now though! Commented Sep 1, 2022 at 21:41