In “NB 08/05/2016 – Hackberry: A Gem of a Weed,” Robert & Karen Benson wrote:
I recently read Lab Girl (2016) by Hope Jahren. She recounts doing research on hackberry seeds. She discovered that she could dissolve away part of the hard seed coat with acid. The dissolvable mineral was calcium carbonate, as is found in many seashells and limestone. A lattice-like framework of some other mineral remained. Using an X-ray diffraction machine, she determined the other mineral was opal! Opal is a mineral (some geologists say it is a mineraloid) made of hydrated amorphous silicon dioxide; it has microscopic spheres of silica aggregated into layers surrounded by water molecules. The orderly pattern of the silica spheres diffract the light and give precious opals that lovely “play of color” we think of when we think of opals. Although the hackberry opal is not gemstone quality opal, it is still fascinating to think of a plant protecting its embryos in seed coats made of rock! And not just any rock, but a mineral that is also used as a gemstone!
Fascinating! Biology is full of surprises and wonders.