[Desafina (OP)]

 A few years ago in Venezuela, parque nacional de Cainama, the tourist guide found and gave me this rocks, he told us that a geologist told him that those could be radioactive but no one believed him. Nowadays I'm very curious of what kind of rock it may be.

[Bill S]

Hi Desafina. Welcome to TNS.

I’m surprised that none of our resident geologists has responded; so, for the time being, perhaps a word or two from an ancient, one time amateur would, at least let you know that you are not being ignored. 

It looks like a ferrous concretion; possibly limonite and/or goethite. Not sure what the central part is, but it will have been the catalyst around which the ferrous layers were deposited.

Don't give up on TNS, it's a much better forum than your initial experience might suggest. 

Stay safe, Bill.

[Zer0]

Greetings Desafina!
🙏
& From All of US, Welcome to TNS SF.
🥳

Umm...i got no clue as to what that is... I'm no Geologist.
☹️

To Mee it looks like a seed of some sort, or maybe even rotten potatoes.
🤭

But yeah, sounds strange...the Guide just  handed it over to You?
No money charged eh?
🤔

& You simply took it...knowing it's Radioactive?
😲

Bravado!!!
👻

No Worries, someone out here would surely figure this out for ya.
👍


P.S. - @Bill S
HeeY Bill!
🙋
Where have You been?
😑
Hope all's Well?
🤞
Welcome Back!
😊

[Bass]

Looks like a goethite concretion. Iron is leached from elsewhere in the rock and commonly forms concretions in sedimentary rock. These may have originally been pyrite concretions.

[Spamname 250921]

Hi
There are many stones in the world, some of which are very valuable and precious. You need to take this stone to the laboratory where they can do all the tests to determine if the stone is radioactive.

[Just thinking]

Nowadays I'm very curious of what kind of rock it may be.

I am most probably wrong but it looks like a contaminated agate.