I found a mineral near the hexagonal rock formations in High Island, Hong Kong that I'd like to get ID'd. It has a greasy and subadamantine lustre. It can scratch a U.S. Quarter Dollar and some types of glass. Some types of glass are able to scratch the mineral. It is translucent but with a yellow tint, with a tetragonal form. The front is flat, with some cleavages. Its streak is a white-yellow. The back is smooth and bumpy. The sides have a mark with an earthy red colour that may be an impurity.

Here is the front side:

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Here is the back side:

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Here is the side:

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The distance between the 2 little marks is 1mm.


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  • $\begingroup$ Feels like apatite but could be anything $\endgroup$ – Gimelist Nov 10 '18 at 21:44
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    $\begingroup$ Please read our guide for asking "identify this rock" questions and add as much information as you can. $\endgroup$ – Spencer Nov 11 '18 at 1:05
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    $\begingroup$ 1. I've added all the info I can. I know that the specific gravity is over 1.36 (i.e. The highest density of salt-water). 2. I saw some reddish rocks. If it helps the hexagonal columns are "acidic rhyolite ¾ dacite porphyroblastic lava, rich in potash feldspar and quartz phenocrysts."More on the hexagonal columns can be found here: geopark.gov.hk/en_s4f7.htm 3. I could try but due to its size I don't think it will be of much use. $\endgroup$ – Kyky Nov 11 '18 at 2:14
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    $\begingroup$ Glass is less useful as an identifier than it used to be, thanks to newer higher strength glasses. see if it will scratch bottle glass and mild steel. $\endgroup$ – John Nov 11 '18 at 18:30
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    $\begingroup$ The guide neglected to say that the choice of background is important (so I updated it). A polished granite countertop makes it hard to make out details in the rock itself. Thus do we all learn. $\endgroup$ – Spencer Nov 12 '18 at 14:59

Given its collection location, near rhyolitic columnar basalt, its somewhat resinous luster, typical of weathered feldspar crystals (Cut it would be vitreous), I would suggest that you have an orthoclase feldspar crystal. In effect you have one of the potash feldspar phenocrysts described in your comments. These crystals and related plagioclase feldspar phenocrysts are common in areas with weathered basalts. For the most part they have little worth unless the crystal is solid and contains some sheen, in which case it can be used as a gemstone, see moonstone. It seems like Wikipedia has one of the easier to read descriptions. Wikipedia description of orthoclase

Your sample is very similar to some sunstones, plagioclase feldspar crystals, I have collected Oregon, but your description of the host rock suggests the orthoclase variety of feldspar. Both types of feldspars can look similar and have similar properties.


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