The name of the emerald filler known as Arthur Groom-Gematrat is being changed to ExCel. The name change came after the product's maker, Clarity Enhancement Laboratory (C.E.L.) in New York, improved it. Introduced in the United States in 1995 by Arthur Groom, president of Arthur Groom-Gematrat and co-founder of C.E.L., the enhancement doesn't change color, doesn't leak out of the stone, and can be removed by the laboratory. In 1997, the laboratory added a UV fluorescent tracer to Gematrat to help gemologists identify the filler and help determine how much was present in the stone. As technologies progressed, so did the challenge of making a better product that could still be identified. ExCel is a much better product, says Groom, but two changes may be obvious to the retail jeweler: The Groom name is gone, and so is the tracer. According to Groom, the reason for eliminating the trace