Spider veins are tiny, thin veins that are seen close to the surface of the skin. They are usually red, blue or green in color and resemble spider webs or marble. They are caused by a backup of blood, which causes the blood vessels to dilate below the surface of the skin. Spider veins are very small varicose veins and it’s important to get them looked at before they worsen. As spider veins are a symptom of venous insufficiency, treatment is often covered by all major insurance carriers.