The isotopic ratios used to make absolute determinations of the formation time of materials are those that involve radioactive isotopes with known decay rates. Here's why:

* Radioactive decay: Radioactive isotopes undergo a predictable process of decay, transforming into other elements or isotopes over time. The rate of this decay is constant and can be measured.

* Half-life: The time it takes for half of the radioactive atoms in a sample to decay is called the half-life. Each radioactive isotope has a specific and constant half-life.

Examples of isotopic ratios used for absolute dating:

* Carbon-14 (¹⁴C) / Carbon-12 (¹²C): Used for dating organic materials up to around 50,000 years old.

* Potassium-40 (⁴⁰K) / Argon-40 (⁴⁰Ar): Used for dating rocks and minerals, particularly those older than 100,000 years.

* Uranium-238 (²³⁸U) / Lead-206 (²⁰⁶Pb): Used for dating very old rocks and minerals, even billions of years old.

* Rubidium-87 (⁸⁷Rb) / Strontium-87 (⁸⁷Sr): Another method for dating rocks and minerals.

Important Note: While these ratios provide absolute ages, there are limitations and assumptions that need to be considered when interpreting the results. For example, the accuracy of dating depends on the initial amount of the radioactive isotope and the preservation of the sample over time.

Let me know if you want more details about any specific method or have further questions.