Understanding Natural Stone Types

Why group these materials together? Marble, limestone and travertine are different stones, but they share many practical maintenance characteristics because they are predominantly carbonate-based materials. They are generally acid-sensitive, relatively soft compared with granite and quartzite, and can often be refinished using similar restoration methods.

Geologically, however, they are not the same. Limestone is a sedimentary rock, travertine is a type of limestone formed by mineral precipitation, and marble is a metamorphic rock produced when limestone or other carbonate stone is subjected to heat and pressure. These differences contribute to the wide range of textures, densities, porosities and appearances found within the group.

All three are commonly used for floors, bathrooms, wall installations and countertops. Marble's recrystallized structure often allows it to develop excellent clarity and depth when polished, while limestone and travertine can range from very dense materials to much more open-grained varieties.

Travertine deserves a special mention from a maintenance standpoint. Its natural voids, fills and visual variation can make minor etching, scratches and traffic wear less obvious than they would be on a very uniform polished marble. That does not make travertine maintenance-free, but it can make normal wear easier to live with.

Marble, limestone and travertine are generally sensitive to acids because carbonate minerals—particularly calcite—make up a large part of their composition. Everyday products such as vinegar, citrus, wine and acidic cleaners can react with the surface and leave dull etch marks.

The severity and speed of the reaction vary with the mineral composition and finish of the individual stone. Dolomitic stones, for example, may react more slowly than calcite-rich materials, but they should still be considered acid-sensitive.

Calcite, the principal mineral in many marbles and limestones, has a Mohs hardness of about 3. Dolomite is somewhat harder, and individual stones may also contain other minerals, so the hardness of the finished stone can vary.

Because these materials are relatively soft, grit tracked in from outdoors can gradually scratch and abrade floors. Quartz, a common component of sand and soil, has a Mohs hardness of 7 and is easily capable of scratching calcite-based stone. Entrance mats and regular dust-mopping are therefore particularly useful for marble and limestone floors.

Porosity varies considerably within this group. Some marbles are very dense and absorb relatively little moisture, while limestone and travertine can range from low-absorption materials to highly porous stones. Even two stones sold under the same general family name can behave very differently.

For that reason, sealing should be based on the actual absorption characteristics of the stone and how the surface is being used rather than on a fixed schedule. Countertops, showers and other areas exposed to water, oils or contaminants should be checked periodically and resealed when the stone begins absorbing liquids readily.

Penetrating sealers can reduce absorption and help protect against staining, but they do not waterproof the stone or prevent etching.

Marble, limestone and travertine are generally very workable materials from a restoration standpoint. Their relative softness allows damaged surfaces to be mechanically ground, honed and polished without the difficulty associated with much harder stones such as granite and quartzite.

Scratches, etches and many forms of surface wear can often be isolated and blended into the surrounding finish, although the ease of a localized repair depends on the particular stone, pattern and level of finish. These materials can also usually be converted between polished, Patina and honed finishes when the installation is suitable.

Natural fissures, filled areas, strong veining and variation in mineral composition can make some repairs more challenging, so repairability still varies from one stone to another.

Granite has long been one of the most common natural stones used for kitchen countertops, and for good reason. It is generally hard, highly resistant to abrasion and available in a very wide range of colours and patterns. It is also commonly used for flooring, exterior cladding and other applications where durability is important. The Natural Stone Institute describes granite as one of the hardest dimension-stone groups and notes its long history in heavily used and exterior applications.

Geologically, true granite is an igneous rock composed primarily of quartz and feldspar, along with smaller amounts of minerals such as mica. In the stone trade, however, the term “granite” is also used commercially for several other hard, silicate-based stones that behave similarly in fabrication and use.

From a maintenance standpoint, granite is usually much less sensitive to everyday acids and scratching than marble or limestone. Individual varieties can still differ considerably in absorption, mineral composition, natural fissures and surface characteristics, so the specific stone should always be considered rather than assuming every granite behaves identically.

In our field experience, we have never encountered the kind of everyday acid etching on true granite that is common with marble and limestone. Granite is generally highly resistant to household acids because its primary minerals are silicates such as quartz and feldspar rather than calcium carbonate.

That does not mean every material sold as granite will behave identically. Commercial stone names can cover a broad range of materials, and surface treatments, resins, fillers or unusual mineral inclusions can sometimes react differently. For that reason, we consider granite highly acid-resistant rather than completely etch-proof.

For normal residential use, exposure to foods such as citrus, wine or vinegar is generally far less of a concern on granite than it is on marble, limestone or other carbonate-based stones.

Granite is composed of several minerals rather than having one fixed Mohs hardness. Feldspar, one of its major components, has a hardness of approximately 6, while quartz has a hardness of 7. Other minerals within the stone may be softer.

In practical terms, this gives most granites excellent scratch and abrasion resistance compared with marble, limestone and other softer carbonate stones. That durability is one reason granite performs so well on countertops, floors and exterior surfaces.

Granite generally has relatively low water absorption, but the amount varies between individual stones. The Natural Stone Institute reports water absorption values of approximately 0.05% to 0.40% by weight for stones within the granite group, illustrating that even materials within the same broad family do not behave identically.

Some granite benefits from a penetrating sealer, while other dense varieties may absorb so little that sealing provides limited additional benefit. Rather than sealing automatically on a fixed schedule, the surface should be assessed for absorption and treated when necessary.

A simple water test can provide a rough indication: if clean water left on the surface causes the stone to noticeably darken, the material is absorbing moisture and may benefit from sealing.

Granite can be repaired and refinished, but its hardness makes the work considerably more demanding than restoring marble or limestone. Chips and cracks can often be filled and blended, while scratches, worn areas and damaged finishes may require diamond grinding and progressively finer abrasives to restore the surrounding surface.

Localized refinishing can be particularly challenging because the repaired area must match the clarity, texture and reflection of the factory-polished stone. Different minerals within the granite can also respond differently during polishing.

In our experience, granite generally requires less routine restoration than softer natural stones, but when grinding or refinishing is required, the work is often slower and more labour-intensive.

Natural quartzite should not be confused with engineered quartz products. True quartzite is a metamorphic rock formed when quartz-rich sandstone is subjected to heat and pressure, recrystallizing the quartz grains into a much harder, tightly interlocked structure. It can be exceptionally strong, hard and resistant to abrasion.

Quartzite has become increasingly popular for custom countertops and slab installations because it can combine excellent durability with movement, veining and sometimes translucency more commonly associated with marble. From a restoration standpoint, however, it remains one of the more demanding materials we encounter because of its hardness and the way different minerals within a slab can respond during refinishing.

One of the biggest complications is identification. Some materials sold commercially as “quartzite” are actually softer carbonate stones, including dolomitic marble, and can behave very differently when exposed to acids or abrasives. Appearance alone is not always enough to distinguish them. When durability is an important reason for choosing quartzite, we recommend testing a sample or offcut before purchasing the slabs.

True quartzite is highly resistant to the ordinary household acids that readily etch marble and limestone. Its dominant mineral is quartz rather than calcium carbonate, so foods such as citrus, vinegar and wine should not produce the characteristic acid etching associated with marble.

In our experience, when a stone sold as quartzite develops obvious marble-like etches from ordinary household exposure, its identification deserves a closer look. Some commercially labelled quartzites are actually dolomitic marble or other carbonate-rich materials.

As with granite, we prefer to describe true quartzite as highly acid-resistant rather than absolutely etch-proof. Surface treatments, fillers and minerals other than quartz can behave differently even when the bulk of the stone is quartzite.

Quartz, the dominant mineral in true quartzite, has a Mohs hardness of approximately 7. As a result, true quartzite is extremely resistant to scratching and abrasion compared with marble, limestone and other carbonate stones. Quartzite itself is a rock rather than a single mineral, however, so its exact behaviour can vary with composition and structure.

A useful practical test is whether a fresh edge of the stone can scratch ordinary glass. Quartzite should be capable of leaving a genuine scratch rather than simply depositing a removable mark on the surface. A stone sold as quartzite that cannot scratch glass should be investigated further.

Quartzite porosity varies considerably from one material to another. Some true quartzites are extremely dense and absorb very little moisture, while others can absorb enough liquid to benefit from a penetrating sealer. The stone should therefore be assessed individually rather than assuming that all quartzite is either porous or non-porous.

Where sealing is appropriate, a penetrating sealer can reduce the absorption of oils, water and other contaminants. As with other natural stones, sealing requirements should be based on the behaviour of the actual material rather than on a fixed schedule.

Mineral inclusions, natural fissures and other features within the slab can also behave differently from the surrounding quartz-rich material and may occasionally contribute to localised discolouration or absorption.

Quartzite can be repaired and refinished, but its hardness makes the process substantially more demanding than working with marble or limestone. Scratches and damaged finishes may require aggressive diamond abrasives followed by progressively finer stages to reproduce the surrounding surface.

In our experience, one of the challenges with quartzite is that different minerals within the same slab can respond differently during grinding and polishing. An area that has been refinished on site may therefore require considerable adjustment of abrasives, pressure, speed and polishing technique before its clarity and reflection match the surrounding factory finish.

Chip and crack repairs can present their own challenges. Many quartzites have translucent or crystalline areas where an opaque repair would be conspicuous, so clear or carefully colour-matched epoxies may be needed depending on the stone and location of the damage.

Quartzite is therefore very repairable in the right circumstances, but matching an existing finish can be considerably less predictable than it is with marble.

Architectural onyx is a highly decorative, translucent carbonate stone best known for its dramatic banding, colour variation and ability to transmit light. It is commonly used for feature walls, furniture, countertops, reception areas and other applications where the appearance of the material is the main focus. Its translucency also makes it particularly well suited to backlit installations.

Although it is often grouped visually with marble, architectural onyx forms differently and can contain calcite, aragonite or combinations of carbonate minerals. These fine crystalline deposits create the layered patterns and translucency that make the stone distinctive.

From a maintenance standpoint, onyx should generally be treated as a delicate decorative stone. It can develop an excellent high-gloss polish, which tends to show its colour, depth and translucency at their best, but it is also vulnerable to acids, scratching, chipping and natural fissures. For that reason, we most often encounter it in lower-traffic or feature applications rather than heavily used floors or kitchens.

Architectural onyx is generally highly sensitive to acids because it is composed primarily of calcium-carbonate minerals such as calcite or aragonite. Household acids and acidic cleaning products can react with the surface and leave dull etch marks, much like they do on marble and limestone.

Because onyx is usually polished to a high gloss, even relatively small etches can be visually obvious where they interrupt the surrounding reflection. Penetrating sealer can help reduce staining, but it does not prevent this chemical reaction.

The hardness of architectural onyx depends on its mineral composition. Calcite has a Mohs hardness of approximately 3, while aragonite is somewhat harder at roughly 3.5–4. Because individual stones can contain different proportions of these minerals, assigning one exact hardness to every variety of onyx would be misleading.

In practical terms, onyx is still a relatively soft stone and can be scratched by harder mineral grit and abrasives. At the same time, many varieties can be quite brittle and may contain natural fissures or layered structures that make them susceptible to chipping or cracking during fabrication and restoration.

This is an important distinction: hardness and brittleness are not the same property. A material can be relatively soft in terms of scratch resistance and still fracture easily.

Porosity varies between different varieties of onyx, so we do not recommend treating every installation according to a fixed sealing schedule. Some material absorbs liquids readily, while denser varieties may show relatively little absorption.

Where the stone is exposed to water, oils or other contaminants, a penetrating sealer can help reduce absorption and staining. The need for sealing should be determined by the behaviour of the actual stone rather than simply because the material is labelled onyx.

As with marble, sealing does not protect onyx from etching.

Onyx can respond very well to professional grinding, honing and polishing, and many of the techniques used to restore marble can also be adapted to onyx. In our experience, it is capable of producing an excellent high-gloss finish when the material is sound.

The challenge is usually not whether the surface can be polished, but the delicacy of the stone itself. Natural fissures, brittle edges, translucent areas and variations within the material can make grinding and repair more demanding. Chips and cracks may also require clear or carefully colour-matched epoxy so the repair does not appear as an opaque patch within the translucent stone.

Localized refinishing can generally be blended successfully, but the individual material needs to be read carefully during the process because different bands or mineral deposits may respond differently to abrasives and polishing.

Slate is a fine-grained metamorphic stone commonly used for floors, entryways, fireplaces, exterior paving and roofing. Its most recognizable characteristic is its ability to split naturally along thin, relatively flat cleavage planes, producing the textured natural-cleft finish seen on many slate tiles.

From a maintenance standpoint, slate behaves quite differently from marble or limestone. It is generally resistant to household acids, but its scratch resistance, absorption and surface texture can vary considerably depending on the particular slate.

Many interior slate floors are left with their natural texture and treated with a penetrating or colour-enhancing sealer. Some installations have also been coated with topical finishes to produce a darker or glossier appearance. When those coatings deteriorate, much of the restoration work involves cleaning or stripping the existing finish and applying a more appropriate treatment.

Slate is generally much more resistant to ordinary household acids than marble, limestone and other carbonate-based stones. The Natural Stone Institute specifically describes slate as chemically resistant, which is one reason it has historically been used for applications such as laboratory surfaces.

Individual slates can contain different minerals, however, so we would not describe every variety as completely etch-proof. Surface coatings, waxes and sealers may also react to cleaners or solvents even when the slate beneath them is unaffected. Damage to a coating can sometimes resemble an etch because the affected area becomes lighter, duller or uneven.

Slate does not have one exact Mohs hardness because it is composed of several minerals. Common constituents can include quartz, mica, chlorite and other fine-grained minerals, each with different hardness characteristics.

In practical use, slate is generally less resistant to scratching and abrasion than granite or quartzite. The Natural Stone Institute describes slate as one of the softer dimension-stone groups and notes that it is not known for particularly high abrasion resistance, despite being successfully used for flooring and countertops.

Scratches and wear can be especially noticeable on darker, smoother or coated slate, while a naturally cleft or highly varied surface may disguise everyday wear more effectively.

Many quality slates have relatively low water absorption, but absorption varies according to the source and characteristics of the individual material. Slate is tested specifically for water absorption in industry standards, which is another indication that it should not be treated as having one universal porosity.

Whether a slate installation benefits from sealing depends on the material, location and desired appearance. Penetrating sealers can help reduce absorption of oils and contaminants, while colour-enhancing sealers are sometimes used to deepen the stone's natural colour.

A topical coating is a different treatment altogether. If one has been applied and begins to wear, trap dirt or discolour, it may need to be stripped before the slate can be properly cleaned and refinished.

Slate can be repaired, but natural-cleft surfaces present a particular challenge. Grinding away a scratch, chip or high point also removes the original cleft texture, so a mechanically refinished patch may look noticeably smoother than the surrounding stone.

For that reason, localized resurfacing is often less practical on natural-cleft slate than it is on marble or limestone. Much of the slate restoration work we encounter instead involves deep cleaning, removing failed topical coatings, replacing sealers or enhancing treatments, and repairing individual chips or cracks where possible.

Some slate is manufactured with smoother honed or other machined finishes, so it would be incorrect to say that slate can never be resurfaced. The real limitation is matching the original surface texture, particularly when the installation has a natural-cleft finish.