Fig. 15. Common crystal form of fluorite.
Fluorite is much too soft for everyday use in jewelry and because of the low refractive index does not yield brilliant faceted stones. The perfect four-directional cleavage, relative softness, and brittle tenacity of the mineral make it difficult to facet. Faceted stones are seldom seen outside of collections. Cabochons are also difficult to cut from this material, but the rich color obtained is ample reward for the time and care necessary in cutting.
Fluorite occurs at several other localities in Texas, notably in Hudspeth, Brewster, Presidio, Llano, and Burnet counties, but not commonly in gem quality or colors that warrant its use as gem material.
Fossil Wood
Wood that is buried in silica-rich sediments is commonly replaced by quartz, agate, or opal. The wood structure, including a large number of the annular rings, knots, small branches, and bark, may be preserved. This process of replacement by silica is believed to take considerable time. Preservations by other means (see Jet, [p. 22]) are known, but silica replacements are most commonly used as gem materials.
Fossil wood is often used by lapidaries as gem material when mineral replacement preserves the wood structure sufficiently well and when various impurities color the replacement material attractively.
Excellent gem-quality fossil wood ([Pl. I], B) has been found at a great number of localities in Texas. Agatized and opalized wood occurs in great abundance along the outcrops of Eocene and Oligocene strata of the Texas Gulf Coastal Plain. Much of this material is very well suited for cabochons, bookends, and other lapidary uses. The preservation is especially good at numerous localities in Washington, Lee, Fayette, and Gonzales counties, and the variety of colors, such as bluish, gray, brown, red, yellow, and black, makes this material especially sought after by “rock-hounds.” Some of the agatized and opalized wood fluoresces yellow or green under ultra-violet light. The fossil wood is sometimes found as stumps, limb sections, or large trunk fragments, but the great majority of the gem material is found as small broken fragments or stream-rolled cobbles.
Fossil palm wood is by far the most sought after variety because this material displays “eyes” and tube-like structures that yield very attractive cabochons and cabinet specimens. Texas fossil palm wood is highly regarded by cutters from all parts of the country, and this material is thought by many lapidaries to be some of the finest gem-quality fossil wood in the United States.
Gravel pits and river gravels in Live Oak County have produced very fine agatized wood. Although the gem material does not seem to be as abundant in this area as it is in counties to the northeast, the vivid colors and excellent preservation of the fossil wood in Live Oak County have attracted collectors from all over the State. The fossil wood usually occurs as large rounded cobbles in the streams. Much of this material is quite translucent when cut and contains various shades of brown, orange, and red.
The gravels of the Rio Grande have produced some fossil wood in addition to the excellent agate that is also found there. Most of the fossil wood found in these gravels is very well preserved, but the colors are commonly dull shades of brown. Occasional fine red and yellow specimens have been recovered from the Rio Grande gravels, but these are rare.