GEM KNOWLEDGE · LABORATORY NOTES
Moonstone (Tanzania)
A bright white feldspar ring supplied from Tanzania was offered as moonstone. Its flash prompted a closer look at which feldspar was actually present.
Finding: The report identifies the bulk as sodium-rich plagioclase between albite and oligoclase, with minor potassium feldspar. The researcher calls the iridescent material albite-peristerite, not moonstone per se. Calcium was too low for labradorite.
Tested specimen: Sample 92955, ring approximately 5 cm at its widest, report completed March 23, 2023. Tanzania is the supplied locality; the report analyzes composition, not mine origin. A visible flash alone does not settle the mineral identification.
Explore the wider distinction in our forthcoming Journal comparison of moonstone, labradorite, and peristerite.
View the original report
Select a page to enlarge it. Three pages; findings are summarized in accessible text above.
The tested identification is Albite Peristerite (Tanzania).
Understanding the report figures
Use this guide alongside the written findings and the original report transcript. It explains the purpose of each analytical figure and preserves the laboratory’s own figure labels. The interpretations apply to the tested specimen; signals alone do not establish the identity of every similar-looking stone.
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Electron microscope image: This view shows small-scale regions selected for analysis. Its brightness contrast helps distinguish regions with different compositions; it is not a photograph of their natural colors. Numbered spots connect this view to the analyses listed in the report.
Laboratory figure caption: Fig. 2 Backscattered electron image of an area of the specimen shown in Fig.1.
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Elemental spectrum: The peaks record characteristic X-ray signals from elements in the analyzed spot or area. This is an analysis of that location, not automatically of the entire specimen. Any mineral identification or uncertainty named in the laboratory caption is retained below.
Laboratory figure caption: Fig. 3: Energy dispersive spectral analysis of spot 1 in Fig. 2 (albite)
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Elemental spectrum: The peaks record characteristic X-ray signals from elements in the analyzed spot or area. This is an analysis of that location, not automatically of the entire specimen. Any mineral identification or uncertainty named in the laboratory caption is retained below.
Laboratory figure caption: Fig. 4: Energy dispersive spectral analysis of spot 2 in Fig. 2 (potassium feldspar).
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Element map: This map shows where the named elements occur within the analyzed area. Its colors represent the laboratory’s selected element channels, rather than the specimen’s visible colors. The caption below preserves any color key or distribution noted by the laboratory.
Laboratory figure caption: Fig. 5: X-ray map of the area shown in Fig. 2. potassium: purple, sodium: yellow.
Read the original laboratory report as text
The following text and tables are transcribed from the original laboratory document. Figure captions identify the report images; the laboratory interpretations and limitations are preserved.
Laboratory report 1
Analytical Result Report
Sample id: 92955 (ring)
Submitted by: K. Bitner
Date submitted: March 23, 2023.
Date completed: March 23 2023.
Operating conditions: An AMRAY 1830 SEM operated at 25 kV acceleration potential, 26 mm working distance, 15 degrees sample tilt, coated with 250 Å carbon. Using the X-ray fluorescence attachment with 35 kV potential in conjunction with the EDS system.
A Bruker Titan X-ray fluorescence spectrometer was also used and operated with 50, 25 and 10 kV.
Fig. 1: Photograph of the specimen. Approximately 5 cm maximum dimension
Fig. 2 Backscattered electron image of an area of the specimen shown in Fig.1.
Fig. 3: Energy dispersive spectral analysis of spot 1 in Fig. 2 (albite)
Fig. 4: Energy dispersive spectral analysis of spot 2 in Fig. 2 (potassium feldspar).
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Fig. 5: X-ray map of the area shown in Fig. 2. potassium: purple, sodium: yellow.
Table 1: Minor and trace element composition of the specimen. (in wt. %).
| Element | Reported value |
|---|---|
| Calcium | 0.7 |
| Iron | 0.02 |
| Titanium | 0.01 |
Summary of findings: This specimen is interesting (Fig. 1). The bulk of the specimen is between albite and oligoclase (Figs. 2 and 3) and minor potassium feldspar (Figs. 2 and 4). The X-ray map (Fig. 5) shows that the sodic plagioclase is dominant. Trace elements are very minor, and this is reflected in the clean white color. The specimen shows nice iridescence to adularescence. It is not a moonstone per se, but an albite-peristerite. It does not contain enough calcium to make it a labradorite.
Date: March, 2023 Signature