Good and bad rocks

The beautiful and varied rocks of Mull have been one of the biggest influences on me. Even when very young,  I realised that not all rocks were the same. Some made lovely outcrops. Some were just one colour. Others were sparklingly varied.  Some contained interresting crystals. Best of all, some had big garnets, or even small sapphires. From an early age, I had strong views about rocks, and was accustomed to judging them on how much I liked them.

Me, aged about 9 (?) investigating beautiful Moine semi-pelites below Dun a' Gheird, east of Uisken on the south coast of Mull. Note the "hard rock" sized hammer.

Although I have been a professional geologist for nearly 30 years, I still can't help making these kind of judgments. There are individual rocks I like. And there are individual rocks I don't.

These judgements can be applied to a landscape or outcrop, or to a hand specimen, or at a microscopic scale. Ideally, a rock should be pleasing at any scale.

Some things that tend to make for a good rock:

• medium-coarse grain size
• pristine igneous or peak-metamorphic assemblages
• some glass  (but not just glass - crystals needed as well)
• inhomogeneity: drusy cavities, banding, layering, magma-mixing, immiscibility etc
• a nice ringing or musical tone when hit with a hammer
• exotic chemistry

Some things that tend to make for a bad rock:

• fine grain size
• homogeneity
• faults (except pseudotachylites, or where nicely mineralised)
• retrogression
• alteration 
• low-temperature shearing/deformation
• small-scale joints or cracks
• low mechanical strength
• mundane chemistry

Of course, this is quite illogical, unjustifiable and many of my criteria are mutually inconsistent. Low temperature equilibration of higher-temperature assemblages creates our inhabitable world. To say that chemistry is mundane, is just to say that it is common, and therefore really important. However, this is not really the point. The alteration of feldspars may be critically important to our Earth, but it doesn't usually produce nice rocks, and a little part of my brain will thus always regard it as a bad thing.  Is this just me? Does anyone prefer a weathered rock to a fresh one? (Apart from microbes).

I must look out some example images of good and bad rocks.