limit, boundary
垠 centers on a dividing line or edge, especially one marked by earth: a limit, boundary, or border.
垠 combines 土 (earth) with 艮, which likely contributes the sound and the idea of a stopping point or limit. The exact historical development is uncertain.
The left side is earth 土, and the right side 艮 looks like a stopper or a boundary marker. Picture an earthen mound marking the limit of a field.
For ギン, imagine a silver (銀, ぎん) boundary marker stuck in the earth: 銀 -> ギン, and the shiny marker shows the limit.