fab_shape.xml

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
<!--    redesign with your own drawables to fit your taste-->
    <item>
        <shape
            android:shape="oval">
<!--            replace with your own colors-->
            <gradient
                android:type="linear"
                android:angle="45"
                android:startColor="#3399CC"
                android:centerColor="#3700B3"
                android:endColor="#FFFFFF"  />

            <padding android:right="15dp" android:left="15dp" android:bottom="15dp" android:top="15dp"/>

            <size
                android:width="56dp"
                android:height="56dp" />
        </shape>
    </item>

    <item android:gravity="center">
<!--        replace with your own image -->
        <bitmap android:src="@drawable/ic_red_cross"
            android:tint="#FFFFFF"
            android:gravity="center" />
    </item>

</layer-list>