Knew it

A woman is sitting beside a couple of amethyst geodes.

'Are these made in volcanoes?'
'Not directly, no, but there can be a link. The crystals of amethyst gradually grow into cavities in a rock, which can be sedimentary or igneous. Very often, though, the original cavities are related to volcanic activity - they can be bubbles of gas in lava flows. When the lava cools and hardens the pockets of gas are trapped. These little caves can then be filled with mineral growth as the material the crystals need to form and grow is washed through the rock with the movement of groundwater.'
'Ah. I knew it. Because they look like volcanoes, don't they?'