Foil, emboss, and letterpress are pressure. A die meets a sheet, the fibre moves, and metal or ink sits in that dent. You can see it in a raking light: the mark is lower or higher than the paper around it, and the edge of the mark casts a small shadow of its own.
A picture of gold placed on top of a finished object is a different thing. It has no dent, no register to the board, and no thickness. Soft light and a glow make it look expensive in a thumbnail and false on a desk.
Name the pressure before you name the metal. If you cannot say what displaced the sheet, you do not have foil. You have a colour.
How to check it
- Rake a light across the mark. A real impression throws a small shadow. A flat colour does not.
- Look at the edge of the metal. It should stop where the die stopped, not fade out like a glow.
- Ask where the sheet was when the mark was made. Flat, or already a box?
Usage
.foil {
/* A dent, not a drop shadow under a sticker. */
background: linear-gradient(#8a6a32, #c4a15a);
box-shadow:
inset 0 1px 0 rgb(255 255 255 / 0.35),
inset 0 -1px 1px rgb(20 19 14 / 0.45);
}