The speaker describes the path their life or body is taking, and it isn't a gentle one. Sand, ash and ruin are all things that break down, scatter and wear away. Against these the poem sets amber, where a body is preserved, held still and made almost beautiful. The speaker says plainly that their way is not that.
The tone is quiet and matter-of-fact. There's no wailing, just a flat list of what lies ahead. Yet the word "alone" in the second stanza adds a note of loneliness, and it's worth noticing that amber is said to form when a body turns into it "all alone." That could suggest the speaker sees preservation as a solitary, sealed-off fate, while decay at least returns us to the wider world. The title, "The way of some flesh," hints that different bodies meet different ends, and this speaker accepts theirs without complaint. The mood is humble, a little bleak, but also calm and clear-eyed.