This crystal is tiny, a mm only, but it exhibits the satisfying deep blue colour only copper is capable to produce. This is Callaghanite, copper magnesium carbonate, found at Gabbs, Nye County, Nevada, USA
Shiny white 1260 degrees centigrade crawl. Apply thick to get crawl. Thicker gives larger crawl areas, thinner gives smaller crawl areas. Too thin and glaze just goes cloudy white with no crawl present. (Best to use Magnesium Carbonate LIGHT in recipe). Clay body used: Potclays white grogged stoneware.