The text of the Clairvoyance spell says:
When you cast the spell, you choose seeing or hearing. You can use the chosen sense through the sensor as if you were in its space. As your action, you can switch between seeing and hearing.
However, the material component is:
a focus worth at least 100 gp, either a jeweled horn for hearing or a glass eye for seeing
Baesd on this, logically, if I only had the glass eye, I would not be able to choose hearing when I cast the spell, nor would I be able to switch to hearing as an action after the initial casting. However, the spell's text doesn't explicitly mention any such limitation, especially for the "switch as an action" feature. So what would be the correct ruling? Is this limitation implied by the description of the material components?
Best Answer
Yes
The RAW seems pretty clear-cut. You need a jewelled horn for hearing and a glass eye for seeing.
But this is a minor matter. Ask your DM if you can get a combined focus to avoid the need to swap foci. This could be a jewelled horn with an inset glass eye, for example. It would cost 200gp if you have a strict by-the-books DM (though I really cannot see a glass eye costing as much as a jewelled horn, so a more lenient DM may be willing to give you a cheaper cost).