Size:4 mmHotfix Rhinestones are small lightweightt and have a heat activated glue already at its back side. HotFix Crystals are also known as flat back hot fix rhinestones. They are known for their versatility and ease of use. Hotfix Crystal are similar to flat back rhinestones that have strong glue set at their back. When the flat side is heated the glue melts and gets attached to the surface.