Above all very light.
It was practically a mantra in the Cartier workshops during the inter-war period. (Perhaps it still is today.) The goal was to create jewelry so fine that mounted stones would appear to float unencumbered by their settings. Cartier's Diamants mystérieux from 1934 were both a feat of miniature engineering and the dernier cri in faddish accessories from a period obsessed with expensive, impractical novelties. If not for contemporary documentation (including several lovely photographs by Horst P. Horst) it would be nearly impossible to believe that they were made at all. From the September 1934 issue of British Vogue:
Jewellery can produce the most astonishing surprises...An ingenious fitting is fixed behind each stone - the tiniest but safest of platinum clips, rather like a hair-grip, with two prongs resting one against the other. The prongs are curved, making the grip even stronger. One is fixed, the other opens at the touch of a spring; you insert the hair, two or three hairs suffice, and release the spring to close the clip. Cartier suggests setting ten or twelve diamonds in this way and arranging them according to your taste - they can even be worn in the eyebrows...
They were a total flop; or, at least, there are no extant examples. People just weren't inclined to have "ten or twelve" of their best diamonds tied up in eyebrow clips for more than a season or two. Horst's lovely photos, however, have survived. They conjure up a glamorized rendition of Man Ray's already-famous image, Tears, from just a few years prior. A wistful beauty lost in contemplation, her glass tears replaced by glittering brilliants.