The Dactyliotheca is a collection of 388 precious and ornamental stones, often mounted on sheets of agate or other ornamental material, especially with an octagonal composite cut, ranging from 28x18x2 to 40x20x3 mm, or with an oval cabochon cut of 20x15x4 mm. . These foils can be interchangeable on two rings, which are also included in the collection. The gems, of different shapes and cuts, are frequently surmounted by small gems or by insects built with various materials.

The stones are arranged on 12 plateaux, divided into three sections of four plateaux each according to the shape and type. The plateaux can be placed one on top of the other in a wooden cabinet or in three book-shaped containers, bound in red leather, with the inscription Lang's Museum Mineralogicum (Inventory of the University of Rome No 402). Nowadays the Dactyliotheca represents one of the most important collections of gems in the world to the point that it was exhibited at the two main world events in the sector: the special exhibition in Munich (1996) and that of Tucson (Arizona , USA) in 2006.

A detailed study of the Dactyliotheca was published by: Graziani G., (1996) "Periodico di Mineralogia", Rome (1996), 65, 79-204