why do conches have a shiny inside.?

Answer 1

They use what is called "nacre"
The shiny appearance of the nacre is due to the fact that the thickness of the aragonite platelets are about 0.5 micrometres, which is comparable to the wavelength of visible light.
Thats how a oyster makes a pearl, a pearl at first is just dirt.
When dirt enters the inside of the oyster, its like an eyelash in your eye. It rolls it around, on its shiny narce inside, and some many years later it makes a shiny ball of dirt, called a pearl.

Answer 2

The type that you purchase have been polished.