Definitions from Facets of Faith: Reflections of God's Glory
adularescence
ad-yuh-lar-ES-ens
A glowing, cloud-like shimmer in moonstone and other gems, caused by light bouncing between thin layers of rock.
albite
AL-bite
A white or clear feldspar mineral common in many rocks.
amphibole
AM-fih-bohl
A group of silicate (silicon + oxygen) minerals that are found in some rocks, like hornblende.
anisotropic
an-eye-so-TROP-ik
A mineral that acts or looks different (like hardness or color) depending on the direction viewed or traversed.
asterism
AS-ter-izm
A star-shaped reflection on a cabochon-cut gemstone, like a sapphire, caused by light bouncing off tiny needle-like inclusions.
aventurescence
av-en-tur-ES-ens
A sparkly effect in gems, like sunstone, where tiny flat bits (such as mica or hematite) inside reflect light like glitter.
birefringence
bye-reh-FRIN-jens
When a gem splits light into two beams, creating a doubled effect, like in calcite.
botryoidal
bot-ree-OY-dal
A rock or gem with a bumpy surface that looks like a bunch of grapes.
boudins
boo-DANS
Long, sausage-like shapes in rocks formed when hard layers get stretched and separated during rock changes.
brecciated
BRECH-ee-ay-tid
A rock made of broken pieces stuck together, like a natural mosaic.
chatoyancy
shuh-TOY-un-see
A shiny, cat's-eye effect in some gems caused by light reflecting off tiny fibers.
cleavage
KLEEV-ij
The way some minerals, like mica, split cleanly along flat surfaces due to their crystal structure.
clinochlore
KLY-noh-klor
A green, flaky or scaly mineral commonly found in metamorphic rocks like serpentine.
concretion
kon-KREE-shun
A round or flat disc of mineral that forms inside another rock.
cristobalite
kris-TOH-buh-lite
A type of silica mineral found in volcanic rocks and as an element in stones like chalcedony and opal, stable at high temperatures.
cryptocrystalline
krip-toh-KRIS-tuh-lin
Rocks or gems with super-fine crystals that are so indistinguishable from one another that they look smooth to the naked eye.
dendrite
DEN-drite
A tree- or branch-like pattern in a rock or gemstone, often made by manganese or iron.
dichroic
dye-KROH-ik
When a gemstone shows two different colors depending on which angle you view it from.
extrusive
ek-STROO-siv
Volcanic rock that forms when lava cools quickly on the Earth's surface.
feldspar
FELD-spar
A common aluminosilicate mineral group that forms a big part of Earth's crust, often found in rocks like granite.
felsic
FEL-sik
Light-colored volcanic rocks with lots of quartz and feldspar, like granite.
fracture
FRAK-cher
The jagged or curved way a mineral breaks when it doesn't split cleanly, like the curved breaks in obsidian.
igneous
IG-nee-us
Rocks formed when melted rock (magma or lava) cools and hardens.
inclusions
in-KLOO-zhuns
Tiny bits of material (liquid, gas, or solid) trapped inside a rock or gemstone.
luster
LUS-ter
How a gem or mineral shines when light hits it — for example, the glassy glow of quartz, the waxy sheen of chalcedony, or the metallic glint of pyrite.
macrocrystalline
mak-roh-KRIS-tuh-lin
Gems or rocks with crystals big enough to see without a microscope, like amethyst.
metamorphic
met-uh-MOR-fik
Rocks changed by heat or pressure into new forms, like marble.
microcrystalline
my-kroh-KRIS-tuh-lin
Rocks or gems with crystals only visible with a microscope.
mineraloid
MIN-er-uh-loyd
A substance that looks like a mineral but doesn't form crystals, like obsidian.
moganite
MOH-guh-nite
A microcrystalline silica mineral mixed with quartz in gems like agate, found in volcanic rocks.
Mohs scale
MOZE SKAYL
A 1-through-10 scale that shows how hard a mineral is, from soft talc (1) to tough diamond (10).
nodular
NOD-yuh-lar
Rocks or minerals that form in round or lumpy shapes.
opalescence
oh-puh-LES-ens
A gem's milky, rainbow-like shimmer that changes as you move it, like in opals.
orthoclase
OR-thoh-klase
A type of feldspar mineral, often pink or white, found in many rocks.
orthorhombic
or-thoh-ROM-bik
A crystal shape with three sides of different lengths, all at right angles.
pectolite
PEK-toh-lite
A white or gray mineral that forms needle-like or fibrous clumps.
pegmatite
PEG-muh-tite
An igneous rock, usually a type of granite, with extra-large crystals, often containing gemstones.
plagioclase
PLAY-jee-oh-klase
A type of feldspar mineral ranging from white albite to calcium-rich anorthite, found in many rocks.
pleochroism
play-AWK-roh-izm
When a gemstone shows different colors depending on which angle you view it.
pseudomorph
SOO-doh-morf
A mineral that looks like one type but has morphed into a different one.
scalenohedral
skal-uh-no-HEE-dral
A crystal shape with eight or twelve triangular faces, like a stretched pyramid or double pyramid, often seen in calcite crystals that look like sharp dog's teeth.
schiller effect
SHIL-er ef-EKT
A shiny, glowing look in gems, like a light coming from inside, caused by layers or plate-like bits reflecting light.
sedimentary
sed-ih-MEN-tuh-ree
Rocks formed by bits of dirt, sand, or shells piling up and experiencing extreme pressure.
silicate
SIL-ih-kate
A mineral group containing silicon and oxygen, often combined with other elements like aluminum or magnesium, forming the majority of Earth's crust and many gemstones like quartz or garnet.
spodumene
SPOD-yoo-meen
A mineral with long crystals. A popular source for commercial lithium, it occurs as gemstones like kunzite.
stromatolitic
stroh-MAT-oh-lit-ik
Layered rock structures made by ancient microbes trapping sediment — like fossilized pancakes in old ocean rocks.
tenebrescence
ten-uh-BRES-ens
A color change in certain minerals, such as hackmanite, where exposure to ultraviolet light or sunlight darkens the color, which fades back to the original color in darkness — like a reversible suntan.
vitreous
VIT-ree-us
A shiny, glass-like appearance in a mineral or gemstone.
vug
VUG
A small cavity in a rock, often lined with sparkly crystals.