diff --git a/src/main/kotlin/apps/ch13_groups.kt b/src/main/kotlin/apps/ch13_groups.kt
index 15c8447..4fdb6ab 100644
--- a/src/main/kotlin/apps/ch13_groups.kt
+++ b/src/main/kotlin/apps/ch13_groups.kt
@@ -58,7 +58,7 @@ fun main() {
             Cylinder(0, 0.5, true, t)
         }
 
-        Group(gt, m, listOf(concentricCylinder1, concentricCylinder2, concentricCylinder3, concentricCylinder4))
+        Group(gt, m, children=listOf(concentricCylinder1, concentricCylinder2, concentricCylinder3, concentricCylinder4))
     }
 
     val decorativeCylinder1 = run {
diff --git a/src/main/kotlin/shapes/Cone.kt b/src/main/kotlin/shapes/Cone.kt
index c00a7e8..a2481d2 100644
--- a/src/main/kotlin/shapes/Cone.kt
+++ b/src/main/kotlin/shapes/Cone.kt
@@ -13,7 +13,7 @@ class Cone(minimum: Number = Double.NEGATIVE_INFINITY,
            maximum: Number = Double.POSITIVE_INFINITY,
            val closed: Boolean = false,
            transformation: Matrix = Matrix.I,
-           material: Material = Material(),
+           material: Material? = null,
            castsShadow: Boolean = true,
            parent: Shape? = null):
     Shape(transformation, material, castsShadow, parent) {
@@ -21,8 +21,9 @@ class Cone(minimum: Number = Double.NEGATIVE_INFINITY,
     val minimum = minimum.toDouble()
     val maximum = maximum.toDouble()
 
+    // Note due to Kotlin semantics, we have to use objMaterial here.
     override fun withParent(parent: Shape?): Shape =
-        Cone(minimum, maximum, closed, transformation, material, castsShadow, parent)
+        Cone(minimum, maximum, closed, transformation, objMaterial, castsShadow, parent)
 
     override fun withMaterial(material: Material): Shape =
         Cone(minimum, maximum, closed, transformation, material, castsShadow, parent)
diff --git a/src/main/kotlin/shapes/Cube.kt b/src/main/kotlin/shapes/Cube.kt
index e863d3b..9fffca8 100644
--- a/src/main/kotlin/shapes/Cube.kt
+++ b/src/main/kotlin/shapes/Cube.kt
@@ -8,12 +8,14 @@ import math.Intersection
 import kotlin.math.absoluteValue
 
 class Cube(transformation: Matrix = Matrix.I,
-           material: Material = Material(),
+           material: Material? = null,
            castsShadow: Boolean = true,
            parent: Shape? = null):
     Shape(transformation, material, castsShadow, parent) {
+
+    // Note due to Kotlin semantics, we have to use objMaterial here.
     override fun withParent(parent: Shape?): Shape =
-        Cube(transformation, material, castsShadow, parent)
+        Cube(transformation, objMaterial, castsShadow, parent)
 
     override fun withMaterial(material: Material): Shape =
         Cube(transformation, material, castsShadow, parent)
diff --git a/src/main/kotlin/shapes/Cylinder.kt b/src/main/kotlin/shapes/Cylinder.kt
index 4bc9701..4632cd1 100644
--- a/src/main/kotlin/shapes/Cylinder.kt
+++ b/src/main/kotlin/shapes/Cylinder.kt
@@ -11,7 +11,7 @@ class Cylinder(minimum: Number = Double.NEGATIVE_INFINITY,
                maximum: Number = Double.POSITIVE_INFINITY,
                val closed: Boolean = false,
                transformation: Matrix = Matrix.I,
-               material: Material = Material(),
+               material: Material? = null,
                castsShadow: Boolean = true,
                parent: Shape? = null):
     Shape(transformation, material, castsShadow, parent) {
@@ -19,8 +19,9 @@ class Cylinder(minimum: Number = Double.NEGATIVE_INFINITY,
     val minimum = minimum.toDouble()
     val maximum = maximum.toDouble()
 
+    // Note due to Kotlin semantics, we have to use objMaterial here.
     override fun withParent(parent: Shape?): Shape =
-        Cylinder(minimum, maximum, closed, transformation, material, castsShadow, parent)
+        Cylinder(minimum, maximum, closed, transformation, objMaterial, castsShadow, parent)
 
     override fun withMaterial(material: Material): Shape =
         Cylinder(minimum, maximum, closed, transformation, material, castsShadow, parent)
diff --git a/src/main/kotlin/shapes/Group.kt b/src/main/kotlin/shapes/Group.kt
index 58d7d3d..264ba6e 100644
--- a/src/main/kotlin/shapes/Group.kt
+++ b/src/main/kotlin/shapes/Group.kt
@@ -7,17 +7,16 @@ import math.*
 import math.BoundingBox.Companion.MaxPoint
 import math.BoundingBox.Companion.MinPoint
 import math.Intersection
-import kotlin.math.PI
 
 class Group(transformation: Matrix = Matrix.I,
-            material: Material = Material(),
+            material: Material? = null,
             children: List<Shape> = emptyList(),
             castsShadow: Boolean = true,
             parent: Shape? = null):
     Shape(transformation, material, castsShadow, parent) {
 
     // Make copies of all the children to backreference this as their parent.
-    val children = children.map { it.withParent(this) }
+    val children = run { children.map { it.withParent(this) } }
     val size = children.size
     val isEmpty = children.isEmpty()
     val isNotEmpty = children.isNotEmpty()
@@ -28,8 +27,9 @@ class Group(transformation: Matrix = Matrix.I,
     operator fun contains(s: Shape): Boolean =
         s in children
 
+    // Note due to Kotlin semantics, we have to use objMaterial here.
     override fun withParent(parent: Shape?): Shape =
-        Group(transformation, material, children, castsShadow, parent)
+        Group(transformation, objMaterial, children, castsShadow, parent)
 
     fun withTransformation(transformation: Matrix): Shape {
         if (!transformation.isTransformation())
@@ -38,7 +38,7 @@ class Group(transformation: Matrix = Matrix.I,
         return Group(transformation, material, children, castsShadow, parent)
     }
 
-    fun forEach(f: (Shape) -> Unit): Unit {
+    fun forEach(f: (Shape) -> Unit) {
         children.forEach(f)
     }
 
diff --git a/src/main/kotlin/shapes/Plane.kt b/src/main/kotlin/shapes/Plane.kt
index 7ea0764..8e34c2c 100644
--- a/src/main/kotlin/shapes/Plane.kt
+++ b/src/main/kotlin/shapes/Plane.kt
@@ -7,12 +7,14 @@ import math.*
 import kotlin.math.absoluteValue
 
 class Plane(transformation: Matrix = Matrix.I,
-            material: Material = Material(),
+            material: Material? = null,
             castsShadow: Boolean = true,
             parent: Shape? = null):
     Shape(transformation, material, castsShadow, parent) {
+
+    // Note due to Kotlin semantics, we have to use objMaterial here.
     override fun withParent(parent: Shape?): Shape =
-        Plane(transformation, material, castsShadow, parent)
+        Plane(transformation, objMaterial, castsShadow, parent)
 
     override fun withMaterial(material: Material): Shape =
         Plane(transformation, material, castsShadow, parent)
diff --git a/src/main/kotlin/shapes/Shape.kt b/src/main/kotlin/shapes/Shape.kt
index 84b22eb..cfb5d9a 100644
--- a/src/main/kotlin/shapes/Shape.kt
+++ b/src/main/kotlin/shapes/Shape.kt
@@ -9,7 +9,7 @@ import java.util.UUID
 import kotlin.math.PI
 
 abstract class Shape(val transformation: Matrix,
-                     val material: Material,
+                     material: Material? = null,
                      val castsShadow: Boolean,
                      val parent: Shape?,
                      private val id: UUID = UUID.randomUUID()) {
@@ -19,6 +19,14 @@ abstract class Shape(val transformation: Matrix,
                     "\tShape: ${javaClass.name}\nTransformation:\n${transformation.show()}")
     }
 
+    // We need to store the parameter passed in for material here in order to propagate it correctly.
+    // We will access it below: if an object does not have a material, it will try to see if it has a parent
+    // with a material.
+    protected val objMaterial = material
+
+    val material: Material
+        get() = objMaterial ?: (parent?.objMaterial ?: DefaultMaterial)
+
     // This method should only be invoked by Groups containing the object.
     internal abstract fun withParent(parent: Shape? = null): Shape
 
@@ -71,4 +79,8 @@ abstract class Shape(val transformation: Matrix,
     internal val parentBounds: BoundingBox by lazy {
         bounds.transform(transformation)
     }
+
+    companion object {
+        private val DefaultMaterial = Material()
+    }
 }
diff --git a/src/main/kotlin/shapes/Sphere.kt b/src/main/kotlin/shapes/Sphere.kt
index 51d3778..8724dc7 100644
--- a/src/main/kotlin/shapes/Sphere.kt
+++ b/src/main/kotlin/shapes/Sphere.kt
@@ -8,12 +8,14 @@ import math.Intersection
 import kotlin.math.sqrt
 
 class Sphere(transformation: Matrix = Matrix.I,
-             material: Material = Material(),
+             material: Material? = null,
              castsShadow: Boolean = true,
              parent: Shape? = null):
     Shape(transformation, material, castsShadow, parent) {
+
+    // Note due to Kotlin semantics, we have to use objMaterial here.
     override fun withParent(parent: Shape?): Shape =
-        Sphere(transformation, material, castsShadow, parent)
+        Sphere(transformation, objMaterial, castsShadow, parent)
 
     override fun withMaterial(material: Material): Shape =
         Sphere(transformation, material, castsShadow, parent)