Blender
Blender is a free 3D graphics software, which can be used to create an illustration of 3D geometric information, such as particle shapes. Figure 1. shows an example graphical image of hexagonal ice particle with severely roughened surface texture.
Useful information
Brief instruction
Open the application "Blender" and click "File" in the most left of the top tab. Select "Import" and relevant format of the file that contains geometric information. For example, select "Stl". Then, you can select any ".stl" file from your folders. Once select your XXX.stl file, press "Import STL" in the top left of you display.
In your display, a default cube, camera, and an imported geometric medium are shown. In the center of the top header where it shows "blender Render", you can select "Cycles Render" to make any change in a straightforward manner (IMO). In the most top of the right tab, you can choose a medium that you want to make a change of (e.g., camera, cube, your imported file name, etc).
To make changes of a geometry of a medium you choose, press "+" button located at the left edge of the right tab to extend the "Transform" tab where you can modify the location, rotation, scale, and dimension of a medium that you choose.
Choose your XXX.stl file at the most right tab to make further modification of the appearance of the medium. In the second top of the right tab, choose "material" and click "new". Then, several tabs appears including "Custom Properties", Preview", "Surface", Volume", "Displacement", and "Settings". Preview is always helpful to check the current appearance of the medium. In the "Surface" option, for example, if you render an ice crystal, you can choose "Glass BSDF" with some light blue colors. You can also change its "Roughness" and "IOR", a real part of the refractive index to make the graphics to be more realistic. In the "Volume" option, you can choose "Volume Absorption" with a dark blue color for an ice crystal case.
After finish the "Material" setting, choose "Render" to modify camera settings. Choose "Film" to change an exposure of the camera, as well as to check "Transparent" in order to make the background of the image to be transparent (important for making this as an image!). Once complete all the settings, click "Render" in the "Render" tab.
Please give a try with an example .stl file that can be generated with this code.