.button {width:70px;
    height:140px;
    overflow:hidden;
}
.button:after {
    content:"";
    width:100px;
    height:100px;
    background: linear-gradient(left top,#cb60b3 0%,#c146a1 50%,#a80077 51%,#db36a4 100%);
    -moz-transform: rotate(45deg);
    -webkit-transform: rotate(45deg);
    display:block;
    margin-top:20px;
    margin-left:25px;
}

检查小提琴http://jsfiddle.net/sandeep/aZ65c/7/