What is the best material to make a tombstone out of?
I would say the best material from which to make a tombstone is quartzite. This is a metamorphic rock that is made almost entirely of quartz. Quartz is about the most durable mineral there is under surface conditions. Additionally, water would have a very difficult time penetrating the surface of quartzite, harder than it would be for most other stones. So, its chemical stability and resistance to penetration would make quartzite about the best rock there is for any monument that you want to last. Of the types of stone from which one might make a tombstone, marble is probably the worst. Marble is made of the mineral calcite which is quite reactive. Over time, it dissolves fairly quickly, rendering inscriptions illegible. Finally, we must define what is meant by quickly. Under normal condition, it would take centuries for the differences in erosion rates to really become significant. However, if the air in the region is polluted enough the difference could become evident in decades. So,
