Edit in JSFiddle

<div class="box"></div>

.box{
	  width: 250px;
	  height: 250px;
      margin: auto;
	  background: #eee;

	  border: 20px solid transparent;

	  -moz-border-image: -moz-linear-gradient(left, #3acfd5 0%, #3a4ed5 100%);
	  -webkit-border-image: -webkit-linear-gradient(left, #3acfd5 0%, #3a4ed5 100%);
	  border-image: linear-gradient(to right, #3acfd5 0%, #3a4ed5 100%);

	  border-image-slice: 1;
	}