Gary Fish, age 13, of Wichita, Kansas, for his question:

How are emeralds formed?

Two very different minerals are classed as emerald gem stones. One is the true emerald, the other the oriental emerald. Both were formed in igneous rocks such as pegmatite and schist. Probably their molecular ingredients were assembled from hot gases and hot mineral solutions. Both types form hexagonal crystals. However, since their ingredients differ, so do their molecular structures. On the scale of mineral hardness, true emerald rates as 7.5. The oriental emerald is in class nine, second only to the diamond.

The true emerald is a tinted variety of beryl, which is beryllium aluminum silicate. Oriental emerald is a tinted variety of corundum, which is a hard crystal form of aluminum oxide. Massive crystals of beryl may weigh several hundred pounds. However, its vivid green emerald forms are small and rare. They may be more costly than diamonds. But oriental emeralds or corundum are harder, rarer and much more valuable than the so called true emeralds.