<div class='my-circle'></div>

<div class='my-half-circle'></div>

.my-circle {
    position: relative;
    height: 10rem;
    width: 10rem;
    background: linear-gradient(to right, #fff, #000);
    border-radius: 50%;
}

.my-circle::after {
    content: '';
    display: block;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translateX(-50%) translateY(-50%);
    height: calc(100% - 2px);
    width: calc(100% - 2px);
    border-radius: 50%;
    background: #fff;
}

.my-half-circle {
    position: relative;
    height: 10rem;
    width: 10rem;
    background: linear-gradient(140deg, #fff, #000);
    border-radius: 50%;
}

.my-half-circle::after {
    content: '';
    display: block;
    position: absolute;
    top: 0;
    left: 0;
    height: 100%;
    width: 50%;
    background: #fff;
}