Are you sure it is a genuine natural rock? Any suspicion it may be synthetic, man-made?

I was sitting in a restaurant in Florida and all tables had a very similar "rock" table tops. They looked amazing, I thought the colour was that of lapis. After a closer examination I came out with the idea that they were man-made.

Yes, I am sure it is a genuine rock, and not a man-made concoction. I show the photo of a natural weathering surface of this rock, where the harder crystals stand out (Diopside? other minerals?) from the blue matrix.

I found something similar identified as "Sodalite" from the Princess Quarry, Bancroft, Ontario, Canada (see http://www.dakotamatrix.com/products/1764/sodalite), probably from a seam in Proterozoic-aged nepheline syenite, but my rock does not have so much dark crystals as the sample shown.

It should be azurite. In this case, remember that it is a carbonate and could dissolve in acids.

I miss some more data about other constituents of the rock. I’m not able to see this from the very weak photos attached. From the first photo I would speculate that the mineral represents a product of replacement(?) or a matrix mineral as speculated from the second photo(?). What about fluorite? or cordierite in metamorphic rock?, or a kind of amphibole? Mere speculations.

Might it be the rock marketed as "Azul Bahia Granite"? Its appearance varies quite a bit, but it can look like your photos.

You had quite a few suggestions. probably a XRF or XRD may identify the mineral.

I attach the table top from Florida that I mentioned before.

Also, when you suspected diopside I thought wollastonite marble, which can have a similar colour - see second picture.

Dear Roberto ,

I agree with Marcus, the blue mineral could be glaucophane. But a thin section would help for more identification.

Best regards,

Hakima

Dear Roberto,

Is it Covelite?In photos it doesn't have metallic luster. please do XRD, 

Mineral / Rock ID from pictures is notoriously difficult, especially if more "exotic" materials are involved. Our department, the Department of Earth & Atmospheric Sciences Mineral Laboratory at the Metropolitan State University of Denver offers FREE non-destructive & certain destructive mineral identification services as part of our geoscientist training program and university community outreach. It is really free and by processing a sample of your rock you would aid in the training of our geoscience students. Takes usually one semester, but you would receive a fine, detailed report of various test results (XRF, XRD, physical & optical properties, etc.) and interpretations. Next batch of sample analysis starts beginning of February 2017. If you are interested, here is a link with details on the process and how to submit your sample: http://college.earthscienceeducation.net/MIN/MINID.pdf

The mineral appears to be sodalite, by its color and mode of occurence.

Sodalite  can be distinguished from similar minerals by its white (rather than blue) streak.

Sodalite's six directions of poor cleavage may be seen as incipient cracks running through the stone.

It has low specific gravity lower than quartz,  2.2 - 2.3, by which you can distinguish.

It is relatively hard, 5.5 – 6 on the Mohs scale, but  very brittle.

Often fluorescent a creamy-white, yellow, or orange color in shortwave ultraviolet light.

It is often found as vein fillings in plutonic igneous rocks, which the photographs suggest. 

I think the mineral is sodalite as well based on its apparent association with biotite.  I have been to the Princess Sodalite Mine near Bancroft Ontario and have a similar looking specimen sitting on my desk.

The sodalite at Bancroft occurs in veins which cross-cut host nepheline syenite gneiss. Some of the nepheline syenites also host corundum crystals, which could be the positively weathering minerals shown on your photograph.  

Finally, if you have the time, I would encourage you to submit the sample to Uwe's lab above.  If it is sodalite hosted in nepheline gneiss, it would be an excellent specimen for a budding geologist to work on.

OK. The most voted candidate for the blue mineral is Sodalite, and I agree.

The other minerals in this blue rock could be Diopside (the white/clear one) and Aegirina (the dark green to black one). Those minerals are the ones in my opinion that stick out of the naturally weathered surface of the rock, while the blue mineral is the one most wasted by weathering.

Sodalite could be a major component of a Foidolite, i.e. a peralkaline differentiate from K-poor and Na-rich Foid Syenites (the host rock at Bancroft, Ontario and at the quarrying place of Azul Bahia dimension stone in Brazil). Accordingly, the blue rock could be a Tawite, from either Ontario, Canada or Bahia, Brazil.

Some guidance could come from this glossary of alkaline rocks: http://www.umanitoba.ca/science/geological_sciences/faculty/arc/PICTURES/GLOSS.PDF

Even Michael's table top in Florida seems to belong to this amazing family of igneous rocks.

I am going to follow Frederick's suggestion and take a small sample from the piece of rock I have (it is actually a nice ashtray found in a bric-a-brac street market in Rome, Italy) for submission to Uwe’s Department in Denver.

I will keep you posted on this. Many thanks to you all.

Dear Roberto Bencini (Caro Roberto): I'm sure it is a peralkaline sodalite syenite, probably from Bancroft, Ontario, or Pozos da Caldas, Brazil, or Greenland. The blue-violet tinge of the mineral is quite evident in the second photo. But I don´t see in the pictures any aegirine-acmite Na-cpx. which usually forms conspicuos black prismatic or radial-acicular crystals, and biotite is quite scarce too. The other minerals accompanying the sodalite are hard to tell, they could be anything from alkali feldspar to corundum, as such an alkali-rich rock wouldn't contain a Ca-rich cpx, such as diopside. These exotic rocks usually contain just as unusual and rare mineral species, so with simple pictures they are hard to tell. Anyway, being a serious mineral and rock collector, and mineralogist during 50 years, I have never seen such a rock before! Regards, Ciao. Sebastian. 

I cannot resist posting the link to a picture (found today) of a rock of the exact colour, texture and grain size of my specimen. This look-alike is located in the University of Arizona Mineral Museum, has been identified as Sodalite and comes from the Princess Sodalite mine, Bancroft, Ontario, Canada. See http://rruff.info/Sodalite/R040141 .

Dear (Caro) Roberto: The enigma was solved, it is definitely a sodalite syenite from Bacncroft, Ontario, probably the lighter mineral is cancrinite, usually associated with sodalite, being both products of alteration of nepheline, and some K-feldspar could also be present, but it is hard to tell in pictures. These are rare rocks which occur in very few localities around the world, in anorogenic igneous bodies usually a few kilometers in diameter, where sodalite-cancrinite enriched rocks, seams or veins are a very minor part of the total volume of the plutons. I've seen the collection of the Univ. of Arizona in Tucson and that of Univ. of Texas in Austin, they exhibit a lot a rare and important specimens. One of them, at UT-Austin, was really remarkable: a piece of alkali basalt with a mantle nodule of spinel peridotite with transparent peridot olivine, from a modern lava flow in Arizona called the San Marcos Flow. This locality is famous for peridote, almost as much as Zebigret island in the Red Sea, making peridot the "state gem" of the State of Arizona. In that same trip, I left from Austin to Arkansas, to visit the famous Magnet Cove carbonatite -which crops out in a paved road near a town and a nearby creek- and the Mufflesboro kimberlite, where I found some decent mantle nodules dispersed in the soil, with no kimberlite at all. But in a gift shop nearby some mineral specimens were for sale, nothing unusual, the sphalerite-galena ore from Kansas, some gypsum desert flowers, agate and amathyst from Brazil and nice wavellite developed in novaculite, typical of Arkansas..., but what really stroke me was a piece of lava identical to the one I saw two days before in Austin, with the benefit that it was larger and hosted two large mantle nodules of peridotite with vibrant green peridote crystals, and two smaller ones. I recongized it was a sample from the same Arizona lava flow, but it was not for sale, it was only used as bookshelf support! The lady of the shop didn't have the slightest idea of what that sample was, and I managed to buy it from her for just 25 $, and she was very pleased selling it to me... Now it is one of the jewels of my collection, as besides the four mantle nodules -the largest is 9 cm x 5 cm, and has 1 cm transparent yellow-green peridots...,, brown broncite and reddish spinel- there are also in the lava white plagioclase phenocrysts, a xenocryst of mantle pargasite, and also vesicles. It is an outstanding sample, which I show to my students in Petrology classes, even larger and better that the one at the UT Museum, purchased with a few dollars! Un vero colpo di fortuna! Regards (Ciao). Sebastian. 

Initially, I hesitated on a Sodalite diagnosis simply because I am familiar with Sodalite as a collector's mineral, of which I have a nice piece in my rock cabinet, of the intense blue colour variety with thin white veins.  I compared the colours and textures, but the ashtray has a distinct blue-violet tinge, as Sebastiàn says, and a "plutonic" texture. Hence I initially discharged Sodalite, and so I asked the question to Researchgate. Many thanks to all contributors.

Dear Roberto: The intense blue color variety with the thin white weins is the sodalite with hackmanite white veins (a white variety of sodalite) from the Pozos da Caldas Alkaline Complex, in Bahía State, Brazil. I also have such sample, and some decorative "eggs" made of the same rock, purchased in Sao Paulo years ago. Sodalite can be even colorless, it is not always blue or violet, it can have a variety of allochromatic colors, as most silicates and other minerals do, colors which not depend on the presence of colored cations, which otherwise will be idiochromatic, like the deep green and blue of Cu2+ bearing minerals. Regards. Sebastian. 

I attach a photo of my egg and of my rough piece of Sodalite (I now know both come from Bahia, Brazil). Thank, Sebastiàn.

For you researchers interested in highly unusual alkali rocks, I posted another syenite-related question, this one about Charoite (https://www.researchgate.net/post/Is_a_Charoite_rich_rock_found_elsewhere_in_the_world_other_than_in_Siberia)

Dear (Caro) Roberto: In 1984 I assisted the XXVIII iNTERNATIONAL GEOLOGICAL CONGRESS, from august 10-20th, in Moscow's Lomonossov University. It was still the USSR at that time, Premier Brezhtnev opened the Congress in the huge Conference Hall of the Kremlin, with more than 6000 geoscientists of the world over (only four, including myself, were from Venezuela...). I choose to make before and after Congress geological-turistical excursions to Lake Baikal and Ucraine. At Congress end we received boxes with VERY nice mineral samples from all the USSR and a good piece of deep-violet polished charoite, [K(Ca,Na)2Si4O10[OH,F).H2O], which at that time was a popular ornamental stone in the USSR, used to make ashtrays and other gift material which could be purchased at the Beriozka shops. The mineral is not a feldspathoid, it is a rare alteration of nepheline, a sort of phyllosilicate, according to its formula, its color is allochromatic, as it doesn't contain any colored cation . We (some 130 participants) sailed during a whole week along Lake Baikal's western shore in a small boat and could visit the Sludyanka Skarn and the old Zar's lapislazuli mines, and other very interesting geological spots, and incredible "Greenvillian" age outcrops along the Transiberian Railroad track cuts. In the Ucraine, we went by night-train to Kiev, and by bus to the famous Volynisky pegmatite and the Korosten Anorhosite Massif. In all these places we were free to take any samples we wish, either from the abandoned quarries or the dumps. The phlogopite books at Sludyanka were huge, even 1 m thick and wide..., I got a single pseudo-hexagonal crystal (book) some 20 cm in diameter, and a few smaller ones, marialitic lilac scapolite and, of course, some pieces of lapislazuli... In the Ucrainian pegmatite I could get a piece of bicolored topaz limited by two parallel cleavage planes, a fist-sized euhedral microcline, and a strange graphic pegmatite having smoky quartz in creamy-white feldspar, really showy. With regards (Ciao). Sebastian. 

Hi Roberto,

from all the comments above one can learn two things: it is difficult and often impossible to identify rocks and minerals from pictures alone, and sodalite, or a sodalite group mineral, is the best candidate. It looks indeed as an alkaline rock and the grayish, slightly tan phase resembles nepheline. From what I can tell in the picture it is not diopside, but who knows? What about a thin section in the sample? In transmitted light sodalite group minerals are isotropic and have a low refractive index (negative relief), nepheline resembles quartz except for the weak cleavage. As to the weathered surface, both nepheline and sodalite are easily decomposed - it is not clear why sodalite should be more robust, but it might be.

The enclosed two photos show the object which stirred up my question. It is a sort of ashtray, 22 cm long, made out of a piece of nice blue rock. The base of the ashtray corresponds to the weatherd surface (incidentally, I think that the blue mineral, now recognised as Sodalite, is the more weathered one, not in relief on this surface).

One picture is taken in full sun light, and the other in the shade.

There is a converging opinion that this rock comes from the Princess Sodalite mine, Bancroft, Ontario, Canada.

Ciao Roberto: I don't think this ashtray made of sodalite + hackmannite (the white stripes) comes from Canada, it is quite usual to see this sort of things in Brazil, mainly from the Pozos de Caldas alkaline intrusion. These gift items in Brazil are made from all sorts of "hard stones", including citrine or rose quartz, agath, sodalite, amazonite, and the like. The hemispherical cavity in the piece of rock makes it an ashtray (portacenere), I have a similar piece bought in Río de Janeiro made of citrine quartz -or perhaps heat-treated light color amethist, which adquires a caramel-honey color... With regards, Sebastian.

Dear Sebastiàn, I greately appreciated your key contribution last year about Sodalite with white stripes coming from Pozos de Caldas, Bahia, Brasil. I even posted the picture of the samples I have of such Brasilian Sodalite (an egg and a rough piece).

The reason I asked this question about which rock and mineral my ashtray is made of is that I cannot see the typical white strips in the ashtray rock, there is an "intrusive" aspect in it which is not common in the best and purest Brasilian samples, and the colour is not exactly the same blu colour of the Brasilian samples, but more blu-violet.

When I asked the question in ResearchGate, I was aware of truely blue Sodalite (I had some pieces, even if I did not know it was Brasilian), and I was not aware of the Canadian sodalite rock (see http://www.minerals.net/gemstone/sodalite_gemstone.aspx?img=/RoughImage/2/67/Sodalite.aspx#photos and also http://rruff.info/Sodalite/R040141).

Frankly, I think the ashtray rock is much more similar to the blu-violet sodalite rock of the Princess Quarry, Ontario, Canada, than to the white-veined blu Sodalite of the Pozos de Calda intrusion of Brasil.

Kind regards

Roberto