Not all slates are created equal. Slates are rated on 3 levels by the American Society for Testing and Materials (ASTM): S-1 has an estimated lifetime of 75-100 years, S-2 with 50-75 years, and S-3 is so poor that it's not used in roofing. What gives this Metamorphic rock longevity is long term compression. S-2 slates are usually compressed between 200 - 300 million years, and trust me that's still pretty good, it's much better than asphalt shingles. But the true Cadillac of slates is the S-1, "Peach Bottom" slate, which was once quarried in Peach Bottom, PA. This slate belt has been compressed for 500 million years! The quarry has since closed and new "Peach Bottom" slates are not available, which is a shame for such a durable rock! In fact, the "Peach Bottom" slate was deemed the "Best Slate in the World" at the 1851 World Expo in London.
This old Manor in Delaware County PA, built at the turn of the 20th century, was a beautiful sight to see, when I came to inspect it for repair. The 100+ year old slates were dark back, and completely solid. The roof had a handful of slates that have been replaced over the century, and the repair slates were crumbling, while the original roof remained pristine! Look at the texture in this picture:
Here we are performing the repairs with salvaged slates to match. This old world home had "sweep" framing along the eves, making ladder hook access difficult. As you can see in this picture we had to access difficult to reach areas with a safety rope and repelling harness.