Cairns are made of any type of stone that is readily available in the area where they are built.

Here's why:

* Purpose: Cairns are primarily used for navigation, marking trails, or as memorials. They are not constructed with specific types of stone in mind.

* Accessibility: Builders use whatever stones they can find nearby, whether they are granite, limestone, sandstone, or even pebbles.

* Regional Variation: The type of stone used will depend on the geological makeup of the region.

So, to answer your question directly, cairns are not made of any specific type of stone. They are made of whatever stones are available and suitable for stacking.