Rock and Minerals: Procedural Modeling

Rocks, Minerals and Crystals: Procedural Modeling in VFX

Introduction

Crystals and rocks have always been a favorite pass time to model in VFX software. There are a few really great modeling tutorials out there to model crystal growth and procedural mineral systems. I will link a few at the end of this article, but today I wanted to talk about some types of rocks that get overlooked. I also wanted to cover more of the science behind rocks and minerals, as well as their defining features. So let's get started.

What is The Difference Between a Rock and a Mineral?

Minerals are naturally occurring, solid, inorganic substances. The study of minerals is known as mineralogy. People who study minerals are called mineralogists. Minerals are classified according to their physical properties. These include: crystal structure, hardness, color, translucency, and density. Minerals also used to describe different metal types, these types will be listed below. Over 5400 different minerals are known to exist.

Rocks are made up of two or more minerals. They considered compound objects. When they contain valuable minerals they are called Ore. Rocks are also made up of three different categories, which we will describe shortly.

Rule of thumb: rocks can be made out of minerals, but minerals cannot be made out of rocks.

Rock Types

Igneous: These types of rocks are formed by the cooling magma and lava. Igneous rocks form above and below the surface of the Earth. Igneous rocks form to create granular, crystalline rocks. A good example of igneous rock would be Obsidian.

Sedimentary: These rocks are formed when sediments accumulate in one area. These rocks are huge combination of layers of minerals, rock particles or organic materials. Sedimentary rocks are usually formed at the bottom of bodies of water, or under icy surfaces. Limestone would be a good example of a sedimentary rock.

Metamorphic: These rocks are formed through extreme heat and pressure. They are found are found deep within the Earth. Rocks in this category are more dense and compact than other rocks. Fun fact, if these rocks are melted, their resulting output would be igneous rocks. A good example of a metamorphic rock would either be marble or quartzite.

Rocks

Diorite: This rock is classified as an igneous rock. It is composed of granite and basalt. It is found between plate boundaries in the earth's crust or anywhere where the ground has become unstable. You can usually find these rocks in the deep depths of the ocean. Example of this rock: HERE

Diabase: This is another igneous rock. This rock is composed of feldspar and augite minerals. These minerals can form crystalline structures inside the rock, and create some very dusty looking structures. Once again, It can be found between plate boundaries in the earth's crust or anywhere where the ground has become unstable. Example of this rock: HERE

Shale: This is a sedimentary rock. It also fits into a smaller sub-category of rocks called mud-stones. It is made up of many thin layers, and silt and clay-size mineral particles. They can be found in natural gas fields, and oil deposits. Example of this rock: HERE

Sandstone: This is another sedimentary rock. It is one of the most common types of sedimentary rock, and can be found almost anywhere. As the name suggests, this rock slowly becomes sand particles overtime. It can be formed out of granite minerals and quartz. Example of this rock: HERE

Slate: This is a type of metamorphic rock. It is fine-grained and is a combination of shale or mud-stone. it can also be made of clay minerals or micas. It is used in roofing and flooring because of it's durability. Example of this rock: HERE

Manhattan Schist: This is another metamorphic rock. It is composed of gray quartz and white orthoclase minerals. it was formed by compression of the Earth's crust. It is mostly found in the place of it's namesake; Manhattan. Example of this rock: HERE

Mineral Types

Minerals are natural chemicals. They are classified according to their chemistry and crystal form. There are 14 classifications that minerals most commonly fit into.

Native elements.
Oxides.
Hydroxides.
Sulfides.
Sulfates.
Carbonates.
Phosphates.
Halides.
Silicates.
Orthosilicates.
Ring silicates.
Chain silicates.
Sheet silicates.
Framework silicates.

Industrial Minerals: These are minerals that do not contain any metals.

If a mineral is valuable it will contain one the following metal groups:

Base Metals: These are metals that do not contain iron. For example copper is considered a base metal.

Precious Metals: These are metals of high value. For example, gold and iron.

Minerals

Goethite: This is the most common iron oxide mineral. It is a weathering product of multiple iron-based minerals. It is special because it can do something called Pseudomorph with other crystals or minerals. This means it can slowly replace the structure of a mineral overtime with it's own composition. Example of this mineral: HERE

Garnet: This is a large term to describe a group of crystals that have a common crystal structure, and are gemstones. They come in a wide range of colors, and are found throughout the world. They can also exist in all three different rock types. Example of this mineral: HERE

Beryl: This is a rare silicate mineral. It is compound of the element beryllium. It is only found in igneous and metamorphic rocks. Emeralds and aquamarine are one of the two most popular gemstones composed of beryl. NASA also uses beryl to form specialty alloys for their space telescopes. Example of this mineral: HERE

Gold: Gold is both an element and a mineral. It is considered a precious metal because of its attractive color and value. Most gold deposits are used in jewelry making. Example of this mineral: HERE

Quartz: Quartz is one of the most well-known minerals on Earth. It occurs in almost all mineral environments, and helps build many rock formations. Some forms of Quartz, have their color enhanced. Almost all forms of Citrine and amethyst are heat treated. Example of this mineral: HERE

Biotite Mica: Biotite is a name used for a large group of black mica minerals clustered together. They are only found in igneous and metamorphic rocks. It is found in a wide range of crystalline igneous rocks. Example of this mineral: HERE

What is a Crystal?

A crystalline structure(crystal) is formed if the atoms or ions that compose it are arranged in a uniform way. This essentially means, the particles or atoms are arranged in groups of threes, or are not arranged to form compound elements. Crystals are generally formed from minerals. Because the atoms of crystals are formed in groups of threes, they will form into pointy geometric structures. However, crystals are a category that covers more than rocks. Pretty much any material, structure, or solid that fits the chemistry of a crystal can fit in this category as well.

Crystal Types

Her are some categories of crystal formation.

Covalent Crystals: If you have ever heard of a covalent bond in chemistry, these crystals are formations created out of those bonds. These crystals are one gigantic molecule. They are sometimes called Covalent Network Crystals because of how the bonds appear to link across the structure like a network.

Ionic Crystals: These crystals are made of both positive and negative ions. They have high melting points and are non-conductors of
electricity when they are in a solid state. However, they are high conductors when they are a liquid.

Metallic Crystals: These types of crystals conduct heat and electricity. These crystals are made out of positive ions that are surrounded by a field of electrons.

Molecular Crystals: These are made of either atoms or molecules. They are soft in structure, and low melting points. They do not conduct electricity.

Organigenic Crystals: These are crystal structures that are produced by living organisms. This is common in deep sea life forms such as mollusks or other vertebrate creatures.

Liquid Crystal: This describes a state of crystalline matter when it is between a liquid and a solid. For instance, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. They are used in wristwatches, calculators, portable computers, and flat-screen televisions. These types of crystals were so interesting, that it made me consider doing a paper on them later on. (Look out! ;) )

Crystals

Sphalerite: Sphalerite is a zinc sulfide mineral. It can be found in all three types of rocks. It is the most commonly found zinc mineral in the world. It often forms in veins and cavities of caves. Example of this crystal: HERE

Colemanite: This is one of the stable of borate minerals. It also develops an electrical charge when it changes under different temperatures. Example of this crystal: HERE

Amethyst: Amethyst is one of the world's most popular gemstones. Most amethyst deposits are found in in South America and Africa. It is found in all three rock types. Example of this crystal: HERE

Amazonite: This structure is part of the feldspar mineral group. It also contains trace amounts of lead. It was also originally named the amazon stone. Some of the most finest crystalline builds of amazonite are found in Colorado. Example of this crystal: HERE

Ajoite: This crystal is named after the mine it was first found it. Which is the Ajo mine, in Ajo, Arizona USA. It is a rare mineral, and is only found in oxidized copper deposits. Example of this crystal: HERE

Procedural Modeling in Houdini

So now for the fun part! Let's talk about how we can make these rocks and minerals in Houdini. Most tutorials you might find on the structure of rock formation relies on the process of procedural modeling. This is the correct way to build rock as they often have a lot of repeating patterns and overlapping structures. I've linked a few tutorials below, but also keep your eyes peeled for a future tutorial on how to build certain rock types.

Making Procedural Rock Cracks

One way to make cracks inside your geometry is by create a set of overlapping/intersecting objects and laying down an Intersection Analysis SOP. This will create points where the geometry is intersecting, and you can connect the points into lines.

You can also scatter points inside the base geometry for your rock, attach l-systems to the points, randomize the rotation, and then use a subtract boolean with the original object. This will subtract the l-systems that you just made for your rock, and create randomized spaces in them. Or you can merge these systems together.

You could even generate a POP system inside your rock's base geometry and have them move about into a position that best represents your rock's interior. Then time shift them into place, trail them into polygon lines, and have fun merging them with the other elements.

Making Non-Uniform Rocks

Use a mountain SOP....always use a mountain SOP...

If you'd like to create a cluster of rocks that have a some types clustered in one corner and some others in another, start with a base geometry. Maybe a sphere. Then try scattering different points on the sphere based on the Cd attribute. You can use the Attribute Paint SOP to select different areas of the geometry, or the Attribute Noise SOP. You can then copy-to-points your other rock geometry and merge everything together.

You can also you the Boolean SOP to intersect different pieces of geometry together to create gaps, selected faces, or add different elements together.

Try using the default concrete material in Houdini to create more displacement on your rocks. You can go to the displacement setting on the shader, turn it on and dial down the texture. It creates some great lumps and bumps.

Crystals

Crystals are one of the easier rocks to model in Houdini. The best starting point for them are either boxes, or tubes. A tube might sound like a more difficult option to chose, but it can actually give you more control over the sides of your crystal. As well as the option of edging open edges of the geometry to taper into points.

When building a crystal you will probably have to group some edges of your geometry so you can control which face you are extruding and how. Try to make each side of an extended crystal face symmetrical. This will help the rock look more geometric.

When tapering the ends of polygon faces together to make a point for your crystal, make sure you add a fuse SOP at the end. This will fuse any other-lapping points together and close your geometry.

A default glass shader will usually work for diamonds. Just focus on your lights and the reflections going into the diamond and you should be fine for rendering.

Creating Fluorescent Minerals

If you would like to create a stone that appears to glow in the dark, then the best way to approach it is to separate the pieces of the stone so you can render different materials on different parts. For example, if you wanted only the inside of a geode to glow, you would render the inside of the geode as a separate piece inside a different Object Node. This can be easier than just applying materials on the SOP level.

Houdini's default glow material looks fine on rocks. But you will need a non-glowing material on the rock to carry the reflection of the glow.

As stated above, you can create cracks inside your rocks. You can also make these cracks glow. One way to do this is by create a set of overlapping/intersecting geometry and laying down an Intersection Analysis SOP. This will create points where the geometry is intersecting, and you can connect the points into lines.

Play around and focus where you'd like the fluorescent source to be located on your rock. Don't add too much, but rather focus on where the glow is coming from and how you can reflect that.