SAO Yui no Kokoro pendant necklace.
Yui’s Heart (Yui no Kokoro) refers to an item containing the data for Yui’s program, which is around three terabytes large. The item is officially known as MHCP001, which stands for Mental Health Counselling Program, by the system. It has the appearance of a crystalline water droplet.
Pendant size is 4.2 x 2.2cm, and rope length is 48cm (including a 6cm adjustable chain). You can find it in a bracelet version here : Sword Art Online Black Onyx Bracelets.