In focus groups and market research, more brightly colored charms resulted in better sales than did dull or pastel colors. Currently, General Mills conducts "concept-ideation" studies on Lucky Charms. The first boxes of Lucky Charms cereal contained marshmallows in the shapes of pink hearts, yellow moons, orange stars, and green clovers. The lineup has changed occasionally, beginning with the introduction of blue diamonds in 1975, followed by purple horseshoes in 1983, red balloons in 1989, green trees in 1991, rainbows in 1992, blue moons in 1995, leprechaun hats in 1997 (temporarily replaced the green clovers), orange shooting stars and around-the-world charms in 1998 (added blue, green, yellow, purple, and red in 2011), a crystal ball in 2001, and an hourglass in 2008. In 2013, 6 new rainbow swirl moons and 2 new rainbow charms were introduced. From the original four marshmallows, the permanent roster as of 2013 includes eight marshmallows. Older marshmallows were phased out periodically. The first shapes to disappear were the yellow moons and blue diamonds, replaced by yellow/orange pots of gold and blue moons respectively in 1994. In 2006, the assortment included purple horseshoes; red balloons; blue crescent-moons; orange and white shooting stars; yellow and orange pots of gold; pink, yellow, and blue rainbows; two-tone green leprechaun hats; pink hearts (the one shape to survive since the beginning); with the most recent addition being the return of the clovers in 2004. The hourglass shape was retired in spring 2018 and was replaced by a unicorn, which was chosen on social media by way of emojis. The size and brightness of the marshmallows changed in 2004. As of 2019, Lucky Charms have added 3 new unicorns of different colorations. Recent changes to the marshmallows include the star shape taking on a "star" design, the orange five-pointed star being complemented by a white "trail." In late 2005, another marshmallow shape was added, the "Hidden Key". It is a solid yellow marshmallow that resembles an arched door (similar to the shape of a tombstone; flat at the bottom, flat sides with a round top). When liquid is added to the cereal, the sugar in the marshmallow dissolves and the shape of a skeleton key appears "as if by magic." The tagline was, "Unlock the door with milk!"