Diamond Thistle Button.  Available in Bright Silver - non tarnishing, Silver, and Gold.  Sizes: Small (Approx. 3/4" x 5/8") or Medium (Approx. 1" x 6/8").  

Bright Silver - $1.75/each

Silver - $1.60/each

Gold - $2.25/each

Note - Medium is used for dancer's vest.