When customers like Samuel Pepys visited the shop of Thomas Tuttell, instrument maker to the king, they could purchase a pack of mathematical playing-cards. The seven of spades, reproduced as Figure 1, depicted the diverse connotations of magnets, or loadstones. These cards cost a shilling, and were too expensive for many of the surveyors, navigators and other practitioners shown using Tuttell's instruments. They provide an early example of the products promising both diversion and improvement which were increasingly marketed to polite audiences. Tuttell's description of loadstone as ‘a treasure of hidden vertues’ encapsulated many contemporary perceptions of these naturally occurring magnets which were to endure throughout the century. This phrase, with its hints of concealed financial and epistemological benefits, resonates with major eighteenth-century analytical themes, such as commercialization, the opposition between vice and virtue, and the fascination with the occult in the face of Enlightenment rationality. This card is emblematic of the multiple interpretations and utilizations of magnetic phenomena during the eighteenth century. It thus provides a useful starting-point for exploring some of the disputes which arose as enterprising individuals concerned with natural philosophy promoted themselves, their activities and their products.