Crazy -- rocks that resonate and sound like metal!
Unfortunately that article gives zero explanation of how certain rocks can produce resonant tones that sound like metal, but another one does, specifically that the rocks need to be under internal stresses, analogous to a tightened guitar string: [1]
"...it does suggest strongly that the ringing ability is a direct result of internal stresses. ... A possible source of the stresses would likely be the loading stresses from the time when the rock crystallized. ... This "relict stress" theory implies that the ringing rock boulders act much like a guitar string. When a guitar string is limp it does not resonate, but a plucked string will provide a range of sounds depending on the level of applied tension. Likewise, a ringing rock boulder will only emit a dull thud if the boulder is de-stressed; however, boulders will resonate at various frequencies depending on the level of residual stress."
The granite sarcophagus in the "Kings's Chamber" [1] of the Great Pyramid of Giza has been used as a lithophone too. Paul Horn's record album "Inside The Great Pyramid" [2] starts with the sound of the sarcophagus being rung like a bell.
You can buy one from an artist in Maine. He makes them out of reused slate tiles, chipping them down to size for the right resonance. It looks a bit like a xylophone, and sounds very pretty. Looking at his website, he also has rough stone ones as well: https://www.elementaldesign.me/stone-musical-instruments
I would advise watching some of the videos linked near the bottom of the article and then watching some of the others on youtube. Rocks can make beautiful noise
There's a musical stone bridge in County Mayo in Ireland that seems to work similarly enough. You can play it by rolling a stone over the side or by running across it with a stone and dragging it across the top of the walls.
Unfortunately that article gives zero explanation of how certain rocks can produce resonant tones that sound like metal, but another one does, specifically that the rocks need to be under internal stresses, analogous to a tightened guitar string: [1]
"...it does suggest strongly that the ringing ability is a direct result of internal stresses. ... A possible source of the stresses would likely be the loading stresses from the time when the rock crystallized. ... This "relict stress" theory implies that the ringing rock boulders act much like a guitar string. When a guitar string is limp it does not resonate, but a plucked string will provide a range of sounds depending on the level of applied tension. Likewise, a ringing rock boulder will only emit a dull thud if the boulder is de-stressed; however, boulders will resonate at various frequencies depending on the level of residual stress."
[1] https://en.wikipedia.org/wiki/Ringing_rocks#Ringing_ability
[1] https://en.wikipedia.org/wiki/Great_Pyramid_of_Giza#King's_C...
[2] https://www.youtube.com/watch?v=2ZmyGavbgLE&list=PLPtysUSalP...
https://en.m.wikipedia.org/wiki/Bellacorick#History