The trillian shape is a triangular shape of diamond. An adventurous cut of diamond, the "Trillian cut" (triangular brilliant) is notable for its scintillation and remarkable shape. The brilliance of the Trillian is due to the fact that it was developed out of the square shaped "radiant" diamond. The cutting of the radiant into a triangular shape was a technique first developed by Leon Finker in Amsterdam in 1978.

The Trillian cut has a length to width ratio of 1 to 1, as it must be cut to form an equilateral triangular shape, either sharp cornered or slightly rounded cornered. Normally cut with 44 facets (but can be cut with up to 50) it has the capability of producing a brilliant scintillation. Depending on the cut of the facets, the Trillian can serve in a paired form, as a highlight for a centerpiece diamond, or as a centerpiece itself.

The Trillian is also a cut that can vary in look, depending on the cutter and his aesthetic approach. The reason for its preferred use as a companion to a centerpiece diamond is its ability to be cut for varied levels of brilliance. A cut giving a shallow depth can allow its brilliance and fire to be muted, so as not to overwhelm the centerpiece, while a cut for greater depth can bring out its bright scintillation and inner fire, complementing other bright diamond cuts, such as the radiant.

In order to insure the proper mounting for a Trillian, a setting utilizing three V-shaped prongs is required, so as to properly protect the corners. The measurements for the Trillian, as read on a certificate, are for the length, width and depth. These measurements are not made from measuring from tip to tip but through measuring as follows: the width measurement is attained by measuring the distance of the Trillian's longest side to the opposite point (also the height of the diamond); the length is the measurement of the longest side. Pay careful attention to the table, as "windowing", or areas where there is less brilliancy than other areas of the table, can occur in a poorly cut or polished Trillion.