Of these three, only the alpha particles develop halos. Granite includes tiny pieces of biotite, which is a dark mica. Joly found that the halos in that mica were the easiest to observe, because they were the easiest to slice for examination under a microscope. There are many radioactive isotopes in nature, but only three are at the very top of decay chains. We will here especially give our attention to the uranium 238 chain.
Alpha particles make the halos.
Joly discovered it was only the alpha particles which made the halos. The halos are formed where the alpha particles stop. The uranium 238 halos. The half-life clock. There is a clock hidden in each halo.