The following is a Sass mixin for the CSS3 border-image property. It sets an image as the border.

@mixin border-image($source, $slice, $width, $outset, $repeat) {
  border-image-source: url($source);
  border-image-slice: $slice;
  border-image-width: $width;
  border-image-outset: $outset;
  border-image-repeat: $repeat;
  -webkit-border-image: url($source) $slice $width $outset $repeat;
  -o-border-image: url($source) $slice $width $outset $repeat;
}

Here is an example of how-to include the mixin within your Sass files:

@include border-image("border.png", 100%, 1, 0, stretch);