Determining if a rock contains crystals can be done through some simple tests that anyone can perform at home. With a bit of background knowledge, a keen eye, and the right tools, identifying crystals hidden within rocks is an accessible science for amateur geologists and rockhounds alike.
What is a crystal?
A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly, repeating pattern extending in all three spatial dimensions. This internal structure is what gives crystals their characteristic shapes and physical properties.
The word “crystal” comes from the Greek word “krystallos” meaning clear ice or rock crystal. This refers to the clarity and transparency of many crystalline materials like quartz. However, not all crystals are clear or colorless. Many crystals exhibit vivid colors caused by impurities in their atomic lattice structure.
Crystals can form within rocks through the gradual solidification of molten magma or precipitate out of mineral-rich solutions. They may also grow through metamorphic processes that rearrange the internal structure of rocks under heat and pressure.
The characteristic shapes of crystals reflect the specific molecular order and symmetry within the crystal lattice. Common crystal shapes include cubes, prisms, pyramids, rhombohedra, needles, plates, and dendrites.
Identifying crystals in rocks
There are several telltale signs that may indicate the presence of crystals within a rock:
- Glittering or shining surfaces – Light catching on the flat faces or cleavage planes of crystals can produce a sparkly luster.
- Distinct geometric shapes – The characteristic shapes of crystals like hexagons, cubes, and pyramids may be visible protruding from a rock surface.
- Straight lines and smooth flat surfaces – The flat faces and straight edges of crystal growth are clues to look for.
- Colorful patches or bands – Concentrated areas of color may indicate the presence of crystalline minerals.
- Radial clusters or lines – Some crystals exhibit fibrous or radial aggregations that look like bundles or spherulites.
However, the only way to definitively determine if crystals are present is to examine the internal structure of the rock. This can be done by carefully breaking open the rock to expose a fresh surface for inspection.
Tools for identifying crystals
Some basic tools can be useful for crystal identification in rocks:
- Hand lens or magnifying glass – A 10x magnification hand lens allows a closer inspection of a rock’s surface textures and colors.
- Streak plate – Dragging a mineral across a rough ceramic plate leaves a powdery streak of color that aids identification.
- Vinegar – Applying a few drops of vinegar to a mineral sample can indicate the presence of calcite or limestone by fizzing.
- Geology hammer – A rock hammer allows gentle splitting of rocks along cleavage planes to reveal the interior structure.
- Sample vials – Collecting small mineral samples in glass vials helps organize and transport them for further analysis.
Splitting rocks to find crystals
One of the most direct ways to locate crystals within rocks is to carefully break them apart to reveal the crystalline structures inside. Some key tips for splitting rocks successfully include:
- Look for natural cleavage planes or fractures in the rock and aim to break it apart along these weaknesses. Strike the edge of the rock in these spots with the hammer at an angle.
- Place the rock in a sturdy cloth bag or between two other rocks to absorb shock and contain shards when breaking it open.
- Start by gently tapping the rock to develop a fracture, then increase force gradually. Trying to break it too quickly can shatter the crystals.
- Focus on removing small chips, flakes and slices to slowly expose the interior. Avoid pulverizing the rock.
- Smooth newly fractured surfaces by rubbing two rock chips together to better see crystal structures.
With patience, precision, and care, splitting rocks along natural seams can reveal amazing crystalline treasures hidden within. Simply opening the rock in this way often provides the most reliable evidence that a rock contains well-formed crystals.
Identifying specific crystal structures
Once crystals have been exposed in a rock sample, more detailed analysis can determine what mineral species they represent. There are diagnostic properties to look for:
- Crystal shape: The characteristic symmetry and angles of the crystal faces help identify the crystal system. For example, cubic pyrite or rhombic calcite.
- Cleavage angles: The angles between cleavage planes inside a crystal are indicative of the crystal lattice structure. Fluorite, for example, cleaves at 90 degree angles.
- Hardness: The resistance of the crystal to scratching helps determine mineral hardness on the Mohs scale. A fingernail can scratch talc but not quartz.
- Luster: The way light reflects from the crystal surface can be vitreous, silky, metallic or earthy. The luster helps identify the mineral.
- Color: The color of the crystals, streak color, and any color variations can point to particular mineral specimens like purple amethyst or green malachite.
Taking detailed observations and measurements of a crystal sample provides the necessary data to identify the mineral species by comparing to reference guides.
Tools for crystallography analysis
More advanced crystallographic analysis uses specialized tools and instruments to reveal fine structural details:
- Goniometer: Measures interfacial angles and crystal system geometry.
- Petrographic microscope: Examines thin crystal slices with high magnification and polarized light.
- X-ray diffractometer: Fires x-rays into the crystal and determines the diffraction pattern from the atomic lattice.
- SEM/EDS: Scanning electron microscope with energy-dispersive x-ray spectroscopy reveals high-resolution images and elemental composition.
- XRF spectrometer: X-ray fluorescence measurement determines the quantitative elemental make-up of crystal samples.
While these more advanced methods are typically reserved for geological laboratories, they demonstrate that crystals can be analyzed at multiple scales using physics, optics, and chemistry to reveal their full crystalline characteristics.
Conclusion
With a trained eye, basic tools, and proper rock splitting technique, the amateur rockhound can readily identify many types of crystals in rocks. Visually inspecting freshly fractured surfaces and observing crystal shape, color, luster, and hardness provides initial clues. Further analysis can solidly confirm the identity and atomic structure of the crystalline material.
The quest to find crystals hidden within rocks can be extremely rewarding. There are still many discoveries waiting in ordinary looking rocks around us. With diligence and care, the dazzling world of crystals can be revealed.
