4 ways to create rounded Labels in Xamarin.Forms.

CornerRadiusEffect_Droid.cs

using App.Droid.Effects;
using App.Core.Effects;

using System;
using System.ComponentModel;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;
using Android.Graphics.Drawables;
using Android.Util;
using Android.Content;

[assembly: ExportEffect(typeof(CornerRadiusEffect_Droid), "CornerRadiusEffect")]
namespace App.Droid.Effects
{
    public class CornerRadiusEffect_Droid : PlatformEffect
    {
        Drawable orig_Background;

        protected override void OnAttached()
        {
            AddCornerRadius();
        }

        protected override void OnDetached()
        {
            RemoveCornerRadius();
        }

        protected override void OnElementPropertyChanged(PropertyChangedEventArgs args)
        {
            base.OnElementPropertyChanged(args);

            if (args.PropertyName == RoundLabelEffect.CornerRadiusProperty.PropertyName)
            {
                UpdateCornerRadius();
            }
        }
        
        private void AddCornerRadius()
        {
            var view = (View)Element;
            if (view == null) return;

            orig_Background = Control.Background;
            SetBackgroundOnControl();
        }

        private void UpdateCornerRadius()
        {
            SetBackgroundOnControl();            
        }

        private void RemoveCornerRadius()
        {    
            Control.SetBackground(orig_Background);            
        }

        private void SetBackgroundOnControl()
        {
            var radius = (float)RoundLabelEffect.GetCornerRadius(Element);
            var radiusPixels = DpToPixels(Android.App.Application.Context, radius);

            var view = (View)Element;
            var backgroundColor = view.BackgroundColor;
            view.BackgroundColor = Color.Transparent;

            GradientDrawable _gradientBackground = new GradientDrawable();
            _gradientBackground.SetShape(ShapeType.Rectangle);
            _gradientBackground.SetColor(backgroundColor.ToAndroid());
            _gradientBackground.SetCornerRadius(radiusPixels);
            Control.SetBackground(_gradientBackground);
        }

        static float DpToPixels(Context context, float valueInDp)
        {
            DisplayMetrics metrics = context.Resources.DisplayMetrics;
            return TypedValue.ApplyDimension(ComplexUnitType.Dip, valueInDp, metrics);
        }
    }
}

CornerRadiusEffect_iOS.cs

using App.Core.Effects;
using App.iOS.Effects;
using System;
using System.ComponentModel;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;


[assembly: ExportEffect(typeof(CornerRadiusEffect_iOS), "CornerRadiusEffect")]
namespace App.iOS.Effects
{
    public class CornerRadiusEffect_iOS : PlatformEffect
    {
        private nfloat origCornerRadius = 0;

        protected override void OnAttached()
        {
            AddCornerRadius();
        }

        protected override void OnDetached()
        {
            RemoveCornerRadius();
        }

        protected override void OnElementPropertyChanged(PropertyChangedEventArgs args)
        {
            base.OnElementPropertyChanged(args);

            if (args.PropertyName == RoundLabelEffect.CornerRadiusProperty.PropertyName)
            {
                UpdateCornerRadius();
            }
        }

        private void AddCornerRadius()
        {
            origCornerRadius = this.Container.Layer.CornerRadius;
            this.Container.Layer.CornerRadius = (nfloat)RoundLabelEffect.GetCornerRadius(Element);
        }

        private void UpdateCornerRadius()
        {
            this.Container.Layer.CornerRadius = (nfloat)RoundLabelEffect.GetCornerRadius(Element);
        }

        private void RemoveCornerRadius()
        {
            this.Container.Layer.CornerRadius = origCornerRadius;            
        }
    }
}

IconFontLabelRenderer_Droid.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Xamarin.Forms.Platform.Android;
using Xamarin.Forms;
using Android.Graphics;
using App.Droid.Renderers;

[assembly: ExportRenderer(typeof(Xamarin.Forms.Label), typeof(IconFontLabelRenderer_Droid))]
namespace App.Droid.Renderers
{
    public class IconFontLabelRenderer_Droid : Xamarin.Forms.Platform.Android.LabelRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
        {
            base.OnElementChanged(e);

            if (Element == null || Element.Text == null || Element.Text.Length == 0)
                return;

            Typeface tf = null;
            if (IconFontUtils.GetTypeFaceForFontFamily(Element.FontFamily, out tf))
                Control.Typeface = tf;
        }

        static class IconFontUtils
        {
            private static readonly Tuple<string, string>[] CustomFonts = new Tuple<string, string>[]
            {
                new Tuple<string, string>("fontawesome", "Fonts/fontawesome-webfont.ttf"),
            };

            public static bool GetTypeFaceForFontFamily(string fontFamily, out Typeface tf)
            {
                foreach (Tuple<string, string> customFont in CustomFonts)
                {
                    if (fontFamily == customFont.Item1)
                    {
                        tf = Typeface.CreateFromAsset(Xamarin.Forms.Forms.Context.ApplicationContext.Assets, customFont.Item2);
                        return true;
                    }
                }

                tf = null;
                return false;
            }
        }
    }   
}

RoundLabel.cs

using Xamarin.Forms;

namespace App.Core.Controls
{
    public class RoundLabel : Label
    {
        public float CornerRadius { get; set; }
    }
}

RoundLabelEffect.cs

using System.Linq;
using Xamarin.Forms;

namespace App.Core.Effects
{
    public static class RoundLabelEffect
    {   
        public static readonly BindableProperty CornerRadiusProperty =
            BindableProperty.CreateAttached("CornerRadius", typeof(double), typeof(RoundLabelEffect), 0.0, propertyChanged: OnCornerRadiusChanged);

        private static void OnCornerRadiusChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var view = bindable as View;
            if (view == null)
                return;

            var radius = (double)newValue;
            if (radius > 0)
            {
                view.Effects.Add(new CornerRadiusEffect());
            }
            else
            {
                var toRemove = view.Effects.FirstOrDefault(e => e is CornerRadiusEffect);
                if (toRemove != null)
                    view.Effects.Remove(toRemove);
            }
        }

        public static void SetCornerRadius(BindableObject view, double radius)
        {
            view.SetValue(CornerRadiusProperty, radius);
        }
        public static double GetCornerRadius(BindableObject view)
        {
            return (double)view.GetValue(CornerRadiusProperty);
        }

        class CornerRadiusEffect : RoutingEffect
        {
            public CornerRadiusEffect() : base("kvntech.CornerRadiusEffect")
            {
            }
        }
    }
}

RoundLabelRenderer_Droid.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Xamarin.Forms;
using App.Core.Controls;
using Xamarin.Forms.Platform.Android;
using App.Droid.Renderers;
using Android.Graphics.Drawables;
using Android.Util;

[assembly: ExportRenderer(typeof(RoundLabel), typeof(RoundLabelRenderer_Droid))]
namespace App.Droid.Renderers
{
    public class RoundLabelRenderer_Droid : LabelRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
        {
            base.OnElementChanged(e);

            if(e.NewElement != null)
            {
                var label = e.NewElement as RoundLabel;
                if (label != null)
                {
                    SetBackgroundOnControl(label, label.CornerRadius);
                }
            }
        }

        private void SetBackgroundOnControl(RoundLabel view, float radius)
        {   
            var radiusPixels = DpToPixels(this.Context, radius);
            
            var backgroundColor = view.BackgroundColor;
            view.BackgroundColor = Color.Transparent;

            GradientDrawable _gradientBackground = new GradientDrawable();
            _gradientBackground.SetShape(ShapeType.Rectangle);
            _gradientBackground.SetColor(backgroundColor.ToAndroid());
            _gradientBackground.SetCornerRadius(radiusPixels);
            Control.SetBackground(_gradientBackground);
        }

        static float DpToPixels(Context context, float valueInDp)
        {
            DisplayMetrics metrics = context.Resources.DisplayMetrics;
            return TypedValue.ApplyDimension(ComplexUnitType.Dip, valueInDp, metrics);
        }
    }
}

RoundLabelRenderer_iOS.cs

using App.Core.Controls;
using App.iOS.Renderers;
using System;
using System.Collections.Generic;
using System.Text;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer(typeof(RoundLabel), typeof(RoundLabelRenderer_iOS))]
namespace App.iOS.Renderers
{
    public class RoundLabelRenderer_iOS : LabelRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
        {
            base.OnElementChanged(e);

            if (e.NewElement != null)
            {
                var label = e.NewElement as RoundLabel;
                if(label != null)
                {
                    SetCornerRadius((nfloat)label.CornerRadius);
                }
            }
        }

        private void SetCornerRadius(nfloat radius)
        {   
            Layer.MasksToBounds = true;
            Layer.CornerRadius = radius;
        }
    }
}

RoundThingsPage.xaml.cs

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace App.Core.Pages.Other
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
	public partial class RoundThingsPage : ContentPage
	{
		public RoundThingsPage ()
		{
			InitializeComponent ();
		}
	}
}

RoungThingsPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:controls="clr-namespace:App.Core.Controls"
             xmlns:ef="clr-namespace:App.Core.Effects"
             x:Class="App.Core.Pages.Other.RoundThingsPage"
             Title="Round all the things">

    <StackLayout
        VerticalOptions="Center"
        Spacing="20">

        <controls:RoundLabel
            Text="Control"
            WidthRequest="80"
            HeightRequest="80"
            CornerRadius="40" 
            VerticalOptions="Center"
            HorizontalOptions="Center"
            VerticalTextAlignment="Center"
            HorizontalTextAlignment="Center"
            BackgroundColor="Coral" />

        <Label
            Text="Label Effect"
            TextColor="White"
            WidthRequest="80"
            HeightRequest="80"
            ef:RoundLabelEffect.CornerRadius="40"
            VerticalOptions="Center"
            HorizontalOptions="Center"
            VerticalTextAlignment="Center"
            HorizontalTextAlignment="Center"
            BackgroundColor="Aqua" />

        <Frame
            WidthRequest="80"
            HeightRequest="80"
            CornerRadius="40"
            Padding="0"
            VerticalOptions="Center"
            HorizontalOptions="Center"
            BackgroundColor="Red"
            HasShadow="False">
            <Label
                Text="Framed"
                TextColor="White"
                VerticalOptions="Center"
                HorizontalOptions="Center" />
        </Frame>

        <Grid
            WidthRequest="90"
            HeightRequest="100"
            VerticalOptions="Center"
            HorizontalOptions="Center">
            <Label
                Text="&#xf111;"
                TextColor="Green"
                FontSize="95"
                FontFamily="fontawesome"
                VerticalOptions="Center"
                HorizontalOptions="Center"
                VerticalTextAlignment="Center"
                HorizontalTextAlignment="Center" />

            <!-- &#xa;  ==> LINE FEED character -->
            <Label
                Text="Font Icon&#xa;in Grid"
                TextColor="White"
                VerticalOptions="Center"
                HorizontalOptions="Center"
                VerticalTextAlignment="Center"
                HorizontalTextAlignment="Center"
                LineBreakMode="WordWrap" />
        </Grid>
                
    </StackLayout>
</ContentPage>