Perikon detector

Strange as it may seem, the crystal in crystal radios wore out. The rectifier was a contact metal–semiconductor point-contact junction, aka a Schottky diode. I mentioned this once before so I wont get too much into the diode. But suffice it to say that not every part of the crystal would rectify the circuit. Rectification in this case means "act like a diode." More here.
Because the whisker was made of wire or a needle, and the crystal was softer, the surface was worn by use. OK hypothetically not every "crystal" was softer, but they were almost inherently of different harnesses. The very soft Graphite was used as a whisker sometimes. and at the other end, carborundum was considered durable. So either the whisker or the crystal was experiencing wear and usually it was the crystal. So hobbyist chose a crystal and whisker based both on it's sensitivity as a detector but also it's longevity. The hunt was on from the start for a "permanent detector."

Lets look at the hardness of some known whiskers and crystals:
WHISKERS
Graphite: 1-2 Mohs
Bornite: 3 Mohs
Iron: 4 Mohs
Steel: 7-8 Mohs
Bronze: 3 Mohs

CRYSTALS
Galena: 2.5 Mohs
Zincyte: 4 - 4.5 Mohs
Molybdenite: 1 - 1.5 Mohs
Iron Pyrite: 6-7 Mohs
Carborundum: 9 Mohs
Silicium: 7-8 Mohs
Chalcopyrite: 2.5 Mohs

As narrow as that variation is, remember that the Mohs scale is only a 10 point same topping out with diamond. not every whisker and crystal worked well together and even when they did, their mismatched harnesses could be problematic. Some combinations were proprietary and many of those were discovered by Mr. Greenleaf Whittier Pickard. He found both the Zincite /Bornite rectifier and the Zincite / rectifier. He called it the Perikon Detector.

He also tested versions with iron pyrite and silicon crystals. Silicon was novel enough he patented it's use in 1906. His Zinc Oxide detector was patented in 1909. He developed one with Molybdenite the same year. More here.