尘 尖

尘　尖
This, I think, is another example where associated characters also form a phonetic marker


红尘　hong chen common dust i.e. everyman, the mortal (world)
刀尖　dao jian sword point

chen尘 and jian尖 here are half rhymes each has xiao on top. 小土　small dirt = dust.
大小　daxiao = size