This question is requiring a comparison between the bonding in gems and plastic beads, which of course have different natures. Thus, we can conclude that bonding in gems usually ionic because they comprise metal-nonmetal compounds with large electronegativity differences, such as Al₂O₃.
On the other hand, bonding in plastics, in general, tends to be covalent because hydrogen, carbon and oxygen have way similar electronegativities.
In chemistry, the formation of chemical compounds require the appearance of forces able to held atoms together. These forces are called bonds and can be covalent, metallic or ionic depending on the bonding substances. For instance, compounds formed a nonmetal and a metal tend to be ionic, whereas substances formed by two nonmetals tend to be covalent.
In addition, the type of bond defines most of the properties the substance has, thus, ionic bonds lead to solid and molecularly well-defined crystal structures whereas covalent bonds lead to amorphous solids.
In such a way, since gems have gorgeous appearances and are way resistant to high pressures, shear and temperatures, we conclude they have ionic bonds formed between metals and nonmetals.
However, plastic, such as that in plastic beads, will have covalent bonds because it is easily deformed and it is not able to withstand high temperatures, pressures or mechanical shears.
Moreover, the nature of the bonding depends on the electronegativity, which is the tendency an atom has to attract electrons; for that reason, large electronegativity differences lead to the formation of ionic bonds (metals and nonmetals, distant in the periodic table) whereas small differences lead to covalent ones.
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