Druses are small to medium irregular cavities in rocks lined by small, well-formed clusters or aggregates of projecting crystals, usually of the same composition as the enclosing rocks.
These cavities are sometimes filled with water and include vugs, amygdales, geodes, and miarolitic ones.
Also, druses can refer to veins, rock fractures, or mineral surfaces coated or encrusted by these small, projecting mineral crystals.
The coated mineral surfaces usually line cavities or voids within rocks. A good example is fluorite encrusted by dusty or fine-grain overgrowth of milky quartz crystals.
Usually, the tiny projecting sheets or clusters of mineral crystals are parallel with elongated and radiating structures. Also, they are well-formed or euhedral crystals.
On the other hand, drusy or druzy is an adjective and a textural that describes veins, fractures, rock surfaces, or irregular cavities in rocks lined or coated by these small, pointing crystals, i.e., pertaining druses.
Also, druzy may refer to mineral crystals with the crystal habit of growing, tiny projecting crystals. For instance, you can have druzy quartz or amethyst crystal.
Lastly, the term drused means the same as drusy or druzy, while a drusy coating refers to small, pointing mineral crystals.
Druzy geodes, amygdales and vugs
We mentioned that geodes, amygdales, vugs, and miarolitic can be druzy. Here is a brief description of what each is.
1. Druzy geodes
Geodes are hollow, nearly spherical, or globular rock bodies which contain secluded masses or crystals of minerals.
These rock bodies are known as druzy geodes if they contain small, projecting minerals coating their inner surface.
A known example of geode druses is in Rio Grande do Sul, Brazil basalts. The inner surface of these geodes has a banded agate layer encrusted by amethysts.
2. Amygdale druses
Amygdales are small, round, or almond-shaped vesicles or cavities filled partially or fully with secondary mineral crystals. Secondary minerals mean they didn’t crystalize directly from magma melt.
Amygdaloid texture or fabric describes rocks with amygdales.
These cavities may have tiny mineral crystals projecting outwards, forming druzy amygdales.
3. Vugs
Vugs are small to medium irregular cavities lined on the inside by mineral crystals with a composition different from surrounding rocks.
Sometimes, vugs can be lined with small, projecting mineral crystals, forming druzy vugs.
Miarolitic cavities or textures are vugs lined by euhedral minerals formed from the vapor phase. They, too, can be druzy.
How do druses form?
Druses form when minerals crystalize from aqueous fluid or vapor phase that separates from the melt. This separation occurs towards the final stage of melt crystallization and commonly forms pegmatites.
Pegmatites are abnormally large interlocking crystals and are associated with miarolitic textures. They tend to have large cavities towards their core, some drusy.
Also, druses can form from secondary minerals. These minerals are deposited by hydrothermal fluids or underground water, which percolates into voids or cavities within rocks.
Cavities and fissures in rocks can form when fluid exsolve from magma melt, forming gas pockets or tectonic activities like faulting or folding. Also, the dissolution of rocks by solutions can form pores or cavities.
Lastly, as they crystallize, druzy minerals grow well-formed crystals that elongate and project into the cavity’s interior or away from the surface.
Examples of druzy mineral crystals
Druzy crystals commonly encrust veins, fractures, and cavities in igneous rocks, including pegmatites. However, it may also occur in sedimentary and other rocks.
Druzy mineral crystals include apatite, agate, quartz, calcite, geothite, pyrite, opal, malachite, and fluorite. Others are uvarovite, mimetite, pyromorphite, azurite, libethenite, galena, sphalerite, brochantite, descloizite, etc.
These minerals often form small, projecting crystals on their surface. Also, they can coat other minerals.
Here is more on a few of these minerals:
1. Druzy quartz
Quartz is a crystalline silica (SiO2) mineral that is often colorless but can have other colors like pink, orange, dark brown, purple, or black, depending on the impurities present.
When it grows in cavities, it produces well-formed, projecting hexagonal crystals forming a druzy quartz. However, quartz crystals are rare when crystallizing in silica-rich igneous rocks or quartz veins.
Sometimes, druzy quartz crystals can coat chalcedony, chert, calcite, amethyst, etc.
Lastly, abundant druses of quartz occur in Haunted Ridge Rocks in Missouri, USA,
2. Chalcedony
Chalcedony is a translucent or semitransparent semi-precious form of silica with minute crystals and very fine intergrowth of moganite and quartz.
It comes in various colors, mainly grayish to white, grayish blue, or shades of pale brown to nearly black brown.
Sometimes, it can form druses, especially in rock cavities. Several varieties of chalcedony also form drusy fabrics.
3. Druzy agate
Agate is transparent to translucent variety of banded chalcedony available in many colors. However, it can sometimes be patchy or dull.
Druzy agate forms splendid, small pointing crystal that lines cavities in extrusive igneous rocks. Also, it can coat other minerals.
4. Amethyst
Amethyst is a purple or violet quartz variety. It commonly forms druses that line amygdale or geodes. Also, druzy amethysts can line other minerals in these cavities, including the agate.
An example is an amethyst crystal druse in amygdales occurs in Irai, Rio Grande do Sul in Brazil. Here, it encrusts agate.
5. Druzy apatite
Apatite is a transparent to translucent phosphate mineral, usually blue to green. However, it can be colorless, yellow, violet, pink, or brown.
It occurs commonly in granitic pegmatites as an early-stage mineral with quartz, feldspar, and tourmaline.
However, druzy apatite can also occur as a late-stage mineral in pegmatites, vugs, and amygdale or as a miarolitic cavity mineral.
6. Carnelian druzy
Cornelian is a brownish-red, semi-precious variety of chalcedony. Sometimes, it may have druzy quartz crystals within its cavities. Or it can also form this fabric.
7. Apophyllite druses
Apophyllite is often a white or colorless phyllosilicate mineral with a drusy crystal habit. It is common in basalt amygdale or similar rocks and occurs with calcite, zeolites, and other minerals.
In rare cases, it may fill druses in granite metamorphic rocks, syenite fissures, or certain deposits of ores.
8. Druzy pyrite
Pyrite or fool’s gold is a naturally occurring iron sulfide mineral, i.e., iron II disulfide. It has a pale brass-yellowish color that makes it resemble gold.
When in cavities within rocks, it often forms a druzy fabric. For instance, drusy pyrite coats calcite at Campbell Mine in Red Lake, Ontario, Canada.
9. Uvarovite
The bright green, drusy uvarovite crystals make fantastic gems. However, they rarely occur in cuttable sizes. Thus, they are cut into thin pieces for setting in jewelry.
What are the uses of druzy mineral crystals?
Some druzy minerals are valued and pricey gemstones. Their uses include making parts of some jewelry pieces like earrings, rings, bracelets, and pendants for necklaces.
Also, they make ornamental items like towers, cabochons, sculptures, etc., thanks to the appeal.
Lastly, these crystals are a trophy for mineral collectors. Also, some buy them for their metaphysical properties.