Hypabyssal rhyolite porphyry dike (Llanite) is on the left,
In the lower right is a medium-grained Llano granite with faintly blue quartz.
Coin is 17 mm in diameter
this image really doesn't do justice to the actual appearance

(modified 1-1996 and 3-2001, 3-2003, 4-2003) Thanks to Cal, David, Sigrid and Rogan.

A hypabyssal rhyolite porphyry dike (commonly referred to as Llanite) from the Llano Uplift of Central Texas contains phenocrysts of reddish microcline feldspar and blue quartz. The blue quartz phenocrysts are generally euhedral with color zoning from light blue cores to dark blue edges. Quartz phenocrysts retain beta quartz dipyramidal shapes and are up to 5 mm in diameter. The quartz and microcline phenocrysts are enclosed in a microcrystalline matrix of quartz, microcline, plagioclase, biotite, and minor fluorite, magnetite, apatite and zircon [thin section image]. Most of the dike swarm is undeformed (recently deformation has been noted in a sample from the margin). Age is ~1093 Ma (U-Pb zircon, Helper and others, 1996).

The blue color is equally developed in deformed and undeformed samples, which precludes the color being due to deformation. Iddings (1904) described the rock and suggested that the blue color was due to "reflection of blue light-waves from the minute colorless prisms, whose width is a fraction of the length of the light waves". He compares this to Rayleigh scatter in the atmosphere. Burmester and Barker (1970) dissolved a number of the quartz phenocrysts in HF and did XRD analysis on the residue and found zircon. Based on this they also proposed inclusions as the cause of the blue color, but with dispersion in the zircon not Rayleigh scatter as the coloring mechanism. Frazier and Gobel (1982) suggested submicroscopic rutile inclusions might cause the scatter. The most recent study (Coblieg and others, 1986; Zolensky and others, 1988) using STEM and electron diffraction suggested that the blue color was caused by submicrometer-sized ilmenite inclusions. Zolensky and others (1988) also suggest that chatoyance seen in the quartz crystals is the result of reflection off of larger (approx. 0.1 X 1.0 X 20 microns) platy ilmenite crystals and note that the blue quartz is enriched in Fe and Ti. They propose that the smaller ilmenite inclusions are magmatic in origin and not from exsolution.

Zolensky and others (1988) also includes a review and discussion of previously proposed mechanisms for blue color in quartz.

Zircon by XRD or Ilmenite by ED? Given the in situ discovery of the ilmenite inclusions, I'd lean in that direction.

Llanite might not be as unique as has been suggested. See the proof.

The rock (Llanite) is very strong with a crushing strength of 37,800 lb. per sq. inch or 26,577,180 kg per sq. m (Barnes, 1988).

Barnes (1988) also contains directions to a roadcut of the rock. Be careful if sampling, the hardness plus the glassy texture produces some impressive shrapnel when the rock is hammered.

Barnes, Virgil E. (1988) Geological Society of America Centennial Field Guide-- South-Central Section, p.361-368.

Burmester, R.F. and Barker, D.S. (1970) Leaching of Quartz from Precambrian Hypabyssal Rhyolite Porphyry, Llano County, Texas. Contributions to Mineralogy and Petrology, 28, p. 1-8.

Coblieg, Thomas. Zolensky, Michael E.; Sylvester, Paul J. (1986) Why is blue quartz blue?. The Geological Society of America Abstracts with Programs, 18., 6, p. 567

Frazier, A. S., Gobel, V. W. (1982) Rutile as cause of blue color of quartz from llanite, Llano County, Texas. The Geological Society of America Abstracts with Programs, 14, 3. p. 111

Helper, M.A., Gose, W.A., and Roback, R.C. (1996) Virtual geomagnetic pole positions from 1.1 Ga intrusive rocks of the Llano uplift, central Texas. Geological Society of America Abstracts with Programs, 28, 18.

Iddings, J. P. (1904) Quartz-feldspar-porphyry (graphiphyroliparosealaskose) from Llano, Texas. Journal of Geology, 12, p. 225-231.

Zolensky, M.E., Sylvester, P.J., and Paces, J.B. (1988) Origin and significance of blue coloration in quartz from Llano rhyolite (llanite), north-central Llano County, Texas, Am. Min. v. 73, p. 313-323.