As you walk through many older cemeteries in our area and look at the memorials which were placed in the mid to late 1800's, you will notice that you can barely read the engraving. You will also notice that when you touch these old memorials, it feels like touching sandpaper. During that time, memorials were made from limestone, sandstone, and marble. Compared to the wood grave markers which were previously used, these materials seemed very durable.
We now know, however, that these natural stones were sedimentary and metamorphic rock created by organic materials (plants and animals). Over millions of years, these organic materials piled on top of each other and solidified. When placed outside, these materials absorb moisture internally and erode from the inside out. This erosion is called "spalling". Limestone, sandstone and marble have an "erosion factor", or wear away outside at about 1/8" - 1/2" every 75 to 125 years.
Today, we use granite to memorialize our loved ones. Granite was created millions of years ago by volcanic action. Various minerals including quartz, felspar, and mica were melted by the earth's core tremendous heat. It was carried close to the earth's surface where it cooled as a solid mass which now covers much of the earth's surface, just below the soil line.
Granite is an igneous rock and on the 1 - 10 hardness scale, it rates a 7 (compared to a 4 & 5 for the sandstone, limestone and marble). Granite is so hard that it does not absorb enough moisture to "spall", but rather erodes from the outside in, as opposed to inside out. Because of this, we "guesstimate" that granite has an "Erosion Factor" of about 1/10 of an inch every two to five thousand years. Memorials made of granite will last many many generations into the future.