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Rocks for the aquarium

Rocks in the aquarium

I have been getting a number of questions about what type of rocks are best
for use in the aquarium, and I thought I would share this information with
the group, as I am also working on a more detailed article of this subject.

Quartz is an interesting mineral because it comes in many variations.
Literally hundreds. Its principal componant is Silicon dioxide. This is
where your computer chips come from.

These are the main varieties:

 A) Chalcedony- Microcrystalline variety
Scientifically, the term "Chalcedony" refers to any type of microcrystalline
Quartz.  These types include:
Agate - Banded variety
Carnelian - Reddish, transparent to translucent variety
Onyx - Banded variety in which the banding lines are straight and parallel,
and consistent in band size.
Jasper - Opaque variety of Chalcedony that occurs in all colors.
Tiger's Eye - Pseudomorph of Quartz after fibrous Crocidolite.
Chrysoprase - Apple-green variety
Bloodstone (or Heliotrope) - Dark green to greenish blue variety dotted with
small, red, bloodlike spots.
Sard - Brownish to brownish-red, transparent to translucent variety
Sardonyx - Banded variety with straight parallel bands of brownish to red
alternating with white or black bands.
Flint - Massive, uniformly colored variety that is somewhat impure.

B) Amethyst - Purple variety
C) Citrine - Yellow to yellow-brown variety
D) Rose Quartz - Pink variety
E)Rock Crystal - Colorless, transparent variety
F)Smoky Quartz - Brownish-black, "smoky" variety
G)Milky Quartz - White, translucent to opaque variety
H)Rutilated Quartz - Quartz with golden-yellow, needlelike Rutile inclusions
I)Aventurine - Opaque, massive Quartz containing small mica, Hematite, or
Goethite scales which cause a glistening effect.

Again, despite its color or form, (crystal, microcrystalline,
massive,nodular ) its still primarily silica. This will not alter the water
chemistry of your aquarium. Quartz can appear in rocks or Geodes where other
minerals such as Calcite are present, which will alter the pH in water.
Calcite consists of Calcium carbonate, commonly with some impurities of
either iron, magnesium, manganese, and occasionally with zinc and cobalt.
Calicte reacts to acid, so to test for it, simply apply drops of muratic
acid on it and if the rock fizzes, then Calcite is present. Rocks sold as
quartz or black onyx, or crystals  ordinarily are pure quartz mineral, not

I particularly like Onyx and Rose quartz. "Zebra rock", a quartz agate, is
also interesting and commonly sold for aquarium use.

Black Onyx is jet black, shiny and has bands of white. It may have some
spots of dull gray. What I really like about this rock is algae tends not to
grow on black objects! Put a very light colored plant in front of it, such
as E tenellus, and you have quite a sharp contrast

Rose quartz is light pink in color. Deeply colored, massive Rose Quartz is
found in numerous localities in Minas Gerais, Brazil, and the best American
material of this sort is from the Black Hills of South Dakota (near Custer,
Custer Co.).

(Massive - Term used to describe a rock or mineral that has no particular
shape, either because it is non-crystalline
it is composed of tiny, unorganized crystals it is a shapeless fragment of a

Quartz is not particularly heavy. I like to work safely with rocks that can
fit in the palm of my hand. This size generally works well in any size
aquarium for accent solitary aquascaping or for stacking or creating walls.

OK, onto things other than quartz:

Petrified wood:
Petrified Wood is wood chemically replaced by a mineral substance. The
replacement is usually Chalcedony ("silica"), but Opal and other minerals
are also known to replace the wood. When the wood becomes petrified, its
original mold remains intact, but an entire new substance takes the place of
what was once wood. In the Petrified Forest National Monument in Arizona, an
entire forest was transformed into petrified wood. Remains of this ancient
forest can be seen in the huge silicafied logs that are found in the area.

Shale, Slate
Slates are formed from clays, shales, volcanic ash, and other fine-grained
rocks. Minerals present are quartz, sericite, chlorite, some graphite,
titanium oxide, and iron oxides.

Sandstone is a sedimentary rock made of sand-sized grains of minerals,
mostly quartz and feldspar, or older rocks held together by one of several
types of cement or  a fine, muddy matrix. Sandstones vary greatly in color,
composition, texture of grains, degree of cementation and layering. Common
colors include grey, tan and red.

These are just a few examples of the many rocks and minerals that can be
used for decorative purposes in an aquarium. "Pagoda" rock is another,
commonly found in some aquarium stores, but I have not been able to find any
information of its make up. It is quite heavy and has a layered look to it.
I havnt yet done an acid test on it. If anyone has any information on this
type of "rock", please forward it to me.

I hope this information is helpfull. Rocks and shells that definetly will
affect water parameters are limestone and those containing seashell or

Best regards

Robert Paul H
 Quality plants, rocks and wood, and PLEASANT conversation! Now with a live
help line.