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Stone TerminologyContents: General BackgroundOne of the main objectives of this resource is to provide information regarding the specific stone types used in Antiquity and to assist in the physical identification of stone samples recovered from archaeological and historical contexts. This might include material from archaeological or underwater excavations, monuments, historic standing buildings or smaller portable objects. Further applications might include an aid to the identification of diagnostic rock fragments in ceramic thin sections. It is important to stress that this resource is a tool with which one might begin to study stone. It is not and could never be a definitive guide to stone identification. One must also remember that (depending on the nature of the stone) the database may not reveal a precise identification. It will however provide an indication of the type of stone you are investigating or guide you towards various possibilities. If you are interested in establishing a firm provenance for your stone it is still recommended that you examine comparative material and/or have your rock scientifically investigated, whether by petrographic or more sophisticated chemical analysis. LimitationsAs you are no doubt aware it is very difficult (both macroscopically and often microscopically) to pinpoint a stone to its precise geological outcrop. Stones are naturally occurring substances which can be subject to various physical and chemical changes whilst they are being formed, and hence a specific stone type may differ in certain of its physical characteristics, both stratigraphically and also across its outcrop. Ashurst and Dimes (1990), mention that 'as a general rule, sandstones tend to be uniform within a given quarry, whereas limestones may vary markedly in colour, texture and hardness within a 30cm cube'. In light of the above, the project has attempted to include the most typical example of each stone type within the database in addition to any common variations. If you are unsuccessful in your search you might have to consider that you have an atypical example and adjust your search accordingly. It is also important to consider that stones from different parts of the country can be relatively homogenous in appearance and composition. Limestones and some sandstones are good examples of this, often needing the application of sophisticated scientific techniques in order to assign them to their provenance. In this case, the user may return several different possibilities from their stone search; it then depends upon the individual to take the investigation further, whether using more sophisticated identification techniques, specialist advice or further information provided by the database. In addition, it is important to consider that not only can the underlying mineralogical composition of a stone differ slightly within the same outcrop but also physical characteristics can change when a stone is removed from its parent rock. Limestone and chalk, which are typically soft when removed from an outcrop, can harden considerably when exposed for many years to atmospheric conditions. Atmospheric conditions can also weaken the cement holding the grains of limestone together and also change the colour of stone, for example, limestone can change from bright yellow/cream to a grey colour through time. Pollution, lichens, algae and the like can completely mask the original colour of stone, and weathering or burial beneath the soil produce their own unique changes. Even the physical preparation of stones post-quarrying can produce surfaces (dressed, tooled, polished etc.) that can differ considerably from the fresh broken surface of a stone newly taken from its outcrop. In using this database the user should be mindful of these facts and exercise caution. From the above, it is clear that stones frequently do not fit into precise standard categories and the project has attempted (within the relatively rigid constraints of a database) to take this into account in the building of the resource. However, in light of the above, the user has a responsibility to exercise caution and adjust their search accordingly. Used wisely, this resource will supply the user (whether beginner or specialist) with the ability to identify a stone sample and will provide the preliminary information required to lead them onto further study in lithic archaeology. Top of PageIntroduction to the Stone Identification SearchesThis project attempts to provide a searchable relational database, one part of which will allow identification of stone types by searching on various physical characteristics. Since both amateur and professional alike may wish to utilise this facility we have attempted to adopt descriptive terms which will be clear to any user. Hence anyone using this facility should read the classifications and descriptions of the physical characteristics very carefully (these are outlined below and can be found on the 'Search by Physical Characteristics' webpage by clicking the '?' button next to the category selection options). Users should be very familiar with these descriptive terms before starting their search since they may vary slightly from what a geologist might expect them to represent. We have done this so that the beginner can also undertake the identification process quickly and without confusion. The database has a facility that enables the user to enter as much or as little information as is necessary (or that one feels comfortable with). If you are fairly confident of your stone identification abilities then it is recommended that you enter information into the majority of search categories: this will help you pinpoint the stone you are attempting to identify fairly quickly. However if you are uncertain or a new user of the database then we recommend that you make use of the 'Any' category choice. As a result your search will return more possibilities but you are in control of which you accept or reject and these results can then be further narrowed down by adding more descriptive categories to your search. The category selection options are explained below. This information can also be obtained for each category in turn by clicking the '?' button on the related search page. Top of PageStone Surface TreatmentBefore beginning the analysis it is very important that you identify the surface treatment of the sample you are looking at. Ideally it is recommended that the fresh broken surface is the one used to carry out your identification. However, the project appreciates that this is a difficult and often unethical activity particularly when examining an archaeological sample, building or monument so you may have to assess the tooled/dressed or polished surface for your identification. Whichever surface treatment you chose, it must then be the same surface you examine for all the identification tests that follow.
Top of Page ColourColour should be treated with extreme caution when identifying a stone. Colour can vary significantly not just within a single hand specimen but also throughout a particular outcrop even before it has been removed from its parent rock. In addition external factors such as atmospheric conditions, chemical/physical weathering and organic growths can change the colour of a stone and pollution can often completely mask its true colour. Colour is also a very subjective property, to most people, primary colours are well known but any variation from these into intermediate shades can produce problems. For this reason we have used fairly general colour determinations in our database. If you do not return any samples for a particular colour you might wish to try the nearest colour approximation (i.e. cream/buff for white, red/brown for brown). In addition a stone may comprise several different colours, often in the case of many igneous or metamorphic rocks. Where there is more than one predominant colour the user should fill in the 'primary colour' and 'secondary colour' sections with the two most predominant colours in order of precedence. If the stone is predominantly one colour then the 'primary colour' section should be assigned accordingly and the 'secondary colour' section set to 'None'. Munsell Colour codes and descriptions present a whole new range of problems and although they have been assigned to all the stone samples in our collection, it is not possible to make a search on these codes. The issue of colour in photographs is one that we have considered in depth and we have attempted to be consistent as far as is possible. There are various problems for which we have tried to compensate: for example it is particularly difficult to light a sample adequately or to establish proper colour calibration, in thin-section photographs, the content and consistency of the fabric can have widely varying effects on exposure times and true colour representation; and the variation in colour representation in computer monitors is a further problem. Wherever possible, we have established colour calibration so that white is white; nevertheless, not every photograph can be taken as an accurate representation of true colour, but we have followed the Technical Advisory Service for Images guidelines whenever we can. See the TASI webpages on Colour and Resolution Targets and Colour Management for more information. Top of PageSurface AppearanceThe user needs to examine the surface of the stone that have been designated in 'Surface Treatment'. It is important that any user unfamiliar with this terminology reads and fully understands the subtleties of the descriptions provided. It is difficult to fit all stones into these five categories confidently and if you don't know, or are unsure (i.e. the sample might fit two of the descriptions) then specify 'Any/Unsure'. Images demonstrating a typical example of each particular type has been provided by clicking the '?' button.
Top of Page Hardness of the StoneIt was felt that using the Mohs scale of hardness which is adopted by geologists as a test for the hardness of minerals might be misleading to non geologists. Instead identification has been based on four simple categories which could be easily understood and conducted by all. Again one should be aware that if you have an extremely weathered stone (its properties may be different from a fresh example), even the hardest of stones might crumble or detach grains from its outer surface in these circumstances. Always attempt to use a fresh surface.
Top of Page Feel of the StoneThis relates to the 'tactile/feel' you get when a finger is passed over the surface of a stone. Again remember that some stones may have been worn smooth by usage or affected by weathering.
Top of Page Grain SortingSorting refers to the homogeneity (in size) of the grains as a whole in the stone. Images are available to assist the user in visualising the degree of sorting.
Top of Page Grain SizeGrain size is based on the Wentworth-Udden grain size classification scale. To use it, a measurement (by eye) must be made on the predominant size range. If the majority of the grains/crystals are indistinguishable or almost indistinguishable these are classed as 'fine'. If the majority of the grains/crystals range from 0.0625 - 2.0mm and are clearly visible to the naked eye these are classed as 'medium'. If the majority of the grains are larger than 2.0mm in size they are classed as coarse.
Top of Page Reaction to Dilute Hydrochloric AcidYou can test the surface of the stone with a drop of a dilute solution of hydrochloric acid. If there is any calcite present the stone will react by fizzing and giving off a gas. Dolomitic/Magnesian limestone will react with warm dilute hydrochloric acid. Warning: It is recommended that you do not undertake this test unless you are in a laboratory environment. Concentrated hydrochloric acid can cause chemical burns so handle with care and only use a very dilute solution of 10% hydrochloric acid to 90% water. Top of PageStone IdentifiersThese are descriptions which often precede the stone name, i.e. 'oolitic' limestone, 'micaceous' sandstone, 'porphyritic' basalt. A stone is frequently known by one descriptive identifier but you are able to select several if necessary. If you are unsure or do not recognise any of the characteristics just select 'Any'. Images are provided to assist you with your identification.
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