Case of 6 x 10lb Bags - Ventis Gas Log Select White Sand. Dual effect glowing embers is a vermiculite and rock wool mixture used to replace the original material to recreate those like new, realistic glowing embers. Silica sand maximizes diffusion of natural gas in the burner pan. Vermiculite replaces silica sand in propane installations, also increases glow with natural gas. Lava rock (also called volcanic cinders) is used to renew or replace the original rocks that came with the logs. Rock wool provides the bright ember effect with gas logs.