Idioblasts are isolated plant cells which differ from neighboring tissues and contain non-living substances. They have various functions such as storage of reserves, excretory materials, pigments, and minerals. They could contain oil, latex, gum, resin, tannin or pigments etc. Some can contain mineral crystals such as acrid tasting and poisonous calcium oxalate

Calcium oxalate

Calcium oxalate is a chemical compound that forms needle-shaped crystals, known in plants as raphides. A major constituent of human kidney stones, the chemical is also found in beerstone, a scale that forms on containers used in breweries...

 or carbonate or silica.
Any of the tissue or tissue systems of plants can contain idioblasts.
Idioblasts are divided into three main categories: excretory, tracheoid, and sclerenchymatous.

Idioblasts can contain biforine cells that form crystals. The chemicals are excreted by the plant and stored in liquid or crystalline form. In bundles they are known as druse

Druse (botany)

Druse are groups of crystals of calcium oxalate, silicates or carbonates present in plants. Calcium oxalate crystals are found in algae, angiosperms and gymnosperms in a total of more than 215 families. These plants accumulate oxalate in the range of 3%-80% of their dry weight via a...

 and as crystals they can be of raphide [needle] form. When the end of an idioblast is broken the crystals or other substance is ejected by internal water pressure. Ideoblasts of calcium oxalate appear to have no function other than a poisonous deterrent to herbivores.