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So you think limb casts are made at Primary Children's Hospital and have to do with crazy 6-year-old boys bailing out of trees with paper wings strapped to each arm?

Rockhounds will tell you that limb casts are really fossilized branches of trees that bowed in the wind millions of years ago. Dean Richardson, president of the Utah Federation of Gem and Mineralogical Societies, explains how the limb casts were created. "As the limb portion of the trees rotted, the void was filled with minerals. The color depends on the mineral," he said.Richardson is preparing for the 50th anniversary of the Rocky Mountain Federation of Mineralogical Societies, which will be observed in the "Golden Gemboree of the Rockies" show at the Utah State Fairpark this weekend (see box).

Dealers from nine western states will present displays of rough and finished gemstones and minerals. "We'll have dinosaur bone that is 195 million years old and when held up to the light shows blood vessels and the structure of the bone. There will be petrified horn coral found in Woodland near Kamas and even some ancient petrified animal dung called `copralite' from the Morrison Formation," he said.

A special display will feature a faceted topaz of gargantuan proportions: 22,892 carats on loan from the Smithsonian Institute.

Several Western universities will have displays. Silversmithing and gem faceting will be demonstrated and collections of rough and polished materials will be shown.

Geodes and quartz-filled nodules, Brazilian agate and obsidian will catch the eye of visitors. Those who are interested can sign up for four field trips that will be held after the show:

- Topaz Mountain for topaz

- Dugway for geodes

- Francis, Utah, to the Red Horn Coral Claims

- Antelope Springs for trilobites

Keith J. Price is chairman of the show and admits he has collected a few rocks of the "leverite" variety. "You know leverite - you should `leave 'er right' there!"


(Additional information)

`Gemboree' events

What: Utah Federation of Gem and Mineralogical Societies presents the Golden Gemboree of the Rockies, celebrating 50 years of the Rocky Mountain Federation of Mineralogical Societies.

When: Friday and Saturday, June 14-15, 10 a.m. to 9 p.m.; Sunday, June 16, 10 a.m. to 6 p.m.

Where: Fairpark Grand Building, Utah State Fairgrounds, 1000 W. North Temple.

Featuring: Smithsonian Institute's Golden Topaz, 22,892 carats; demonstrations of silversmithing, lapidary, gem faceting; lectures, field trips, exhibits, gems, minerals, rocks, fossils, jewelry.

Cost: Tickets $2.50 at the door, children under 12 free with adult; three-day pass, $6. Free parking.