Natural stones are subject to colour and texture variation - this is what makes them unique, in contrast with man-made materials.
Many natural stones have different grades depending on colour variation and other characteristics. If natural stones are offered at suspiciously low prices, there is likely to be a quality or grade issue. It would be unfortunate to end up with an uninspiring but 'cheap' product - ceramic tiles may have been a better alternative.
Architects and other specifiers are advised to ensure that contractors use the recommended supplier (who provided stones for a project specification) to procure the specified natural stones, to ensure that the stones of the specified quality/grade/quarry are used in the project. This is especially crucial in large projects where different contractors are involved in different stages/areas but the same natural stone has been specified throughout the project. It would be disastrous if different contractors supplied somewhat mismatching stones (eg, different stones, quarries, grades).
Regarding the grade issue. As an example, there are about three to five grades of Crema Marfil, on average. Similarly, Bianco Carrara comes in many grades which command different prices. Thassos is another good example of a natural stone with multiple grades. Thassos AAA is the top grade of this natural stone and about twice as expensive as Thassos First Grade (typically dotted with black spots similar to cigarette burns). Travertine and limestone are also known for variations in quality, etc.
It is important to understand that different quarries produce different stones of the same grade, eg, the Select/Top grade from one quarry may be only as good as the 1st grade from another quarry. The colour/texture/pattern vary from quarry to quarry. Also, the quality of the selection procedures whereby factory workers manually sort the stones into different grades is very important. Luckily, all this is reflected in prices.
Please note that a lower grade natural stone is not only aesthetically inferior but its technical characteristics can be and often are inferior to those of the top/select grade, especially in the case of veiny materials (Crema Marfil, etc.), travertine and limestone. Lower grade natural stones are more susceptible to breakage, can be lacking epoxy filling, have many small holes (especially some types of limestone) or poor filling (travertine).
It is highly recommended that you see the natural stones before buying them. Pictures and small samples can give only a rough idea. Samples are a good starting point but beware of non-representative samples. A visit to the showroom or even the warehouse to see the stones/stocks is highly advisable.
Natural stones should be handled with care when unloading to avoid chipping. In particular, when in a crate, tiles should not be shifted; you should not browse through tiles when they are in a crate because bottom edges of tiles may get chipped; one by one tiles should be lifted out of a crate and stacked on a soft base (eg, a piece of wood) against a wall or other vertical support; stones should not be stacked on the leading edges - only the back edges.
Marble is probably the most luxurious stone. Top quality and less familiar types of marble create the ultimate sense of luxury.
Travertine has been used extensively in many countries, in particular Italy & Persia, both indoors and outdoors. Floors or walls clad in travertine often add Mediterranean flavor to the surroundings.
Limestone varieties come usually in lighter colors and can be quite hard and non-porous similar to marble.
Granite is the toughest of all types of natural stones and is used typically in public areas. It has also become a popular alternative for kitchen worktops, alongside marble worktops.
Onyx is gaining popularity in interior design. Due to its translucent qualities, a back lit onyx wall panel could be a centerpiece in a room. Most popular onyx colors are white and honey.
Natural stones come in various finishes. Broadly speaking, the more popular finish for marble is polished, while for travertine and limestone - honed. In addition, brushed finish goes well with certain types of limestone and travertine.
There are different types of grout, adhesives, sealant and other materials required for fixing natural stones. It is advisable to check with the supplier what materials are best suited for a particular natural stone and types of floor or wall surface.