Question: If someone has some experience with lysozyme crystals I would be interested in knowing whether any of the images below seem likely to be lysozyme. Solutions were made with powdered lysozyme ($\pu{50 mg/mL}$) and $\ce{NaCl}$ ($\pu{3 M}$), except the lower left which is $\ce{NaCl}$ alone.

The top two images were made from a lysozyme solution with a small amount of $\ce{NaCl}$, then adding a piece of tissue paper. The square/cuboid crystals came out of solution immediately onto the fibers. That they are cubes suggest to me $\ce{NaCl}$ but the appearance is odd.

The middle two images have a higher ratio of $\ce{NaCl}$ to lysozyme--maybe close to 2:1 or 1:1. The crystals look OK to me but as a complete novice I am not sure. The crystals are all on the order of $\pu{0.5 mm}$.

[![enter image description here](https://i.stack.imgur.com/te7Z2.jpg)](https://i.stack.imgur.com/te7Z2.jpg)

A better image of the square crystals from lysozyme/$\ce{NaCl}$ + tissue paper.

[![enter image description here](https://i.stack.imgur.com/hkmsf.jpg)](https://i.stack.imgur.com/hkmsf.jpg)

Answer: 
> 
> If someone has some experience with lysozyme crystals I would be interested in knowing whether any of the images below seem likely to be lysozyme.
> 
> 
> 

The bottom two have the typical shape of well-grown lysozyme crystals.
[![enter image description here](https://i.stack.imgur.com/tJAri.png)](https://i.stack.imgur.com/tJAri.png)
Source: Jena Biosciences (<https://www.jenabioscience.com/crystallography-cryo-em/get-started/crystallization-of-model-proteins/co-401-lysozyme-solution>)

Buttonwood's answer about polarized light is spot on.

[![enter image description here](https://i.stack.imgur.com/UbkQo.png)](https://i.stack.imgur.com/UbkQo.png)

You can also add a dye. If it diffuses into the crystal, it can't be NaCl.

There is also a destructive method. If you try to squeeze the crystals with a hard object (like a needle), NaCl crystals will stay intact or break with smooth faces while the lysozyme crystals will look like smashing a watermelon with a baseball bat.

Here are some more ideas: <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2864706/>

And here a paper about dye penetration with some great photos of lysozyme in polarized light: <https://www.semanticscholar.org/paper/Penetration-of-dyes-into-protein-crystals.-McPherson/5d79f2f257f85163d5c14ad0967de1d4354bd1c8>
The one in the top right and those large in the very last photo *pretty much look like* $\ce{NaCl}$. Those in the second row (left) and third row (both left and right) *look different* by the crystal habit's symmetry; and what could be [birefringence](https://en.wikipedia.org/wiki/Birefringence) (especially third row, right hand side) -- a plausible first evidence for protein crystals which typically crystallize in a lower than cubic space group symmetry.

***However:***

The habit of a crystal (outer shape) may vary on the crystal's condition of growth. Only the angles enclosed by the crystal's surfaces remain constant for the same polymorph of a compound (Steno's law). The presence of impurities the purification did not remove, intentionally added detergents, dyes and buffers, the applied technique (e.g., sitting drop *vs.* hanging drop), the age of the crystals while growing may influence which faces may be seen or not (more).

*Remedy:*
A quick and easy check is to observe the sample put between two linear polarizers/Nicols. Check if there is a polarization microscope nearby and put the dish on its table (it very well may contain the solution including the crystals). Since there is there is no birefigence for cubic $\ce{NaCl}$, you may sort these out visually. If Nicols are crossed, crystals of $\ce{NaCl}$ appear uniformly black.

It may happen that accidentally such a salt crystal still ends on the X-ray diffractometer *instead* of your wanted protein crystal. But these will be recognized quickly by symmetry, $d$-spacing and brilliance (compared to a protein sample) of the diffraction pattern.