The plain of Karbala was sacred to Shiah Islam. Locus of the massacre of Husein's small band and of his martyrdom, the site subsequently became a major shrine. Karbala stones were fully described for Westerners early on (1639) by the man from Holstein, Adam Olearius:
The English visitors, George Foster (1783/84) and Lady Mary Sheil, (1849-1853), are two other competent Middle East observers, with this to say, respectively, about the tablets:
An early 20th century mention of the stones appears in d’Allemagne’s extensive work: "...embroidered pieces of linen are used for making little portable prayer rugs, which all good pilgrims put in the bottom of their sacks when they leave on a long trip of sanctification which often lasts several months. In these little portable chapels, there is always a geometric design indicating the direction in which one must put the fabric on the ground, and in the upper part there is a space reserved on which to put the little lump of clay from Kerbala, which all good believers must touch several times with their foreheads when begging Allah for help in their great misery." (4)
The remarks of these four observers link the briquettes with foreheads, give them Mecca directionality, put them on the niche end of prayer rugs, and suggest that there is a spot reserved for their placement. Given these data it seems valid to speculate that representations of the "stones" could appear on prayer rugs. A sound use of this speculation might be to view as a possible tile of Karbala any geometric form, within or above a mihrab, which differs from the other decorative motifs on the rug.
If briquettes are a prayer rug icon, they may be used in establishing provenance, especially in Azerbaijan rugs, for the Azeri population was mixed Shiah and Sunni, with Shiahs in the majority and located principally in areas relatively proximate to what is now Persia, and in the large towns and cities. The Sunnis were both more rural and closer to Turkey. The motif, of course, could have lost its significance with time and thus be no indicator, but with this caveat, some sorting of Caucasian rugs may be possible based on prayer rug analysis.
Another fruitful area may be the Baluch-type prayer rugs which occasionally bear what could be an image of a Karbala briquette. Tribes, Baluch or otherwise, weaving these rugs are likely to have resided in Persian territory.
A related speculation
has to do with the hands appearing in the spandrels of prayer rugs. There
are many symbolic possibilities for hands, but when they appear with a
putative Karbala stone, the urge to consider them the hands of the martyred
Husein is very strong indeed.